MLU FORUM  

Go Back   MLU FORUM > MILITARY VEHICLES > The Armour Forum

Notices

Reply
 
Thread Tools Display Modes
  #1  
Old 18-07-18, 05:58
Malcolm Towrie Malcolm Towrie is offline
Registered User
 
Join Date: Aug 2015
Location: Whitby, Ontario, Canada
Posts: 306
Default Sherman A4A2 diesel 2-3 shift problem

Hi,
We have a Sherman A4A2 with the twin 6-71 Detroit diesels. One engine is slow to drop the rpm when the clutch pedal is pushed down and the throttle pedal is released. This makes the 2-3 shift difficult. I've checked the governor for this engine including weights and all seems to be in good condition. The linkage from throttle pedal to governor seems free, but the rear cross shaft bearings just in front of the rads are worn. Only thing I haven't checked is the low speed spring gaps in the governors. Anyone have any suggestions as to what might cause this slow rpm drop with clutch dipped and throttle released?
Thanks,
Malcolm
Reply With Quote
  #2  
Old 18-07-18, 07:50
tankbarrell tankbarrell is offline
Adrian Barrell
 
Join Date: Mar 2003
Location: Suffolk, UK
Posts: 843
Default

The 2 to 3 shift in Sherman is a problem if you leave the change too late. It's best to just get the vehicle moving in second and then go for third rather than trying to get the speed up in second before changing up.
__________________
Adrian Barrell
Reply With Quote
  #3  
Old 18-07-18, 10:26
David Herbert David Herbert is offline
Registered User
 
Join Date: Mar 2010
Location: Ayrshire, Scotland - previously Suffolk
Posts: 547
Default

Is that engine burning oil ? If so it is possible that the oil is fueling it to keep it going but not enough to over speed it. Look for valve guides, rings and particularly supercharger oil seals.

Synchronising two good engines is possible and when right you cannot really tell that they are two seperate engines. Also the clutch plates last longer as they are not trying to lock two engines together that are doing different things. You just need to follow the instructions in the manual to the letter.

Also check that when you push the clutch down both clutches do fully disengage. One of the joys of M4A2s is that the clutch linkage on the floor gets stuck if the tank gets wet and is not driven frequently.

As Adrian said, change up at the minimum revs that you can but still get the next gear. You will get a much quicker change and the engine has bags of torque so can accelerate from about 1000rpm happily. This is very opposite to a modern car so seems wrong but it is much easier. Running up to maximum revs is fine in heavy going but trying to shift at those revs will result in so much speed loss that you find yourself stopping and starting from scratch. Actually this applies to any WW2 armoured vehicle with a manual shift.

David
Reply With Quote
  #4  
Old 18-07-18, 15:04
Jesse Browning Jesse Browning is offline
Registered User
 
Join Date: Sep 2009
Location: Waterloo
Posts: 366
Default

A Korean War tank mechanic brought up the subject to me as one of the common problems of the day. He said the cause was that the syncromeshs were not worn in yet. He said the solution was to force it into third without the clutch, and after that it would be easy to shift normally. I never had the heart to do that to my transmission, so 2 to 3 remained difficult to the end. The first question the buyer of my Sexton asked me: “what’s the secret to shifting from 2 to 3”? The mechanic also told me they would wire open the governors on the radials on all the vehicles, that the radial engines tanks were much more reliable than the GAA engined tanks, and that they would start the engines on pure alcohol in the winter.
Reply With Quote
  #5  
Old 18-07-18, 20:36
David Herbert David Herbert is offline
Registered User
 
Join Date: Mar 2010
Location: Ayrshire, Scotland - previously Suffolk
Posts: 547
Default

On a firm flat surface you should be able to shift from 2 to 3 after moving about ten feet from start. The higher the revs that you change at the more work the synchromesh has to do and so the longer it takes. The original gearbox oil spec is SAE 50 engine oil. People put thicker oil in them "because it is a big gearbox" and wonder why it won't steer or change gear. Actually a modern 15 - 40 universal plant/agricultural oil works really well and has caused no problems that I know of. The original oil heats up till it is painfully hot to the touch if you run enough which makes it much thinner than a modern SAE 40.

I don't think it is very wise to lock out the governor on a radial engine but the usual way to kill them is to let them idle. They will idle nicely at 500 rpm but at anything under 800 there is very little oil getting to the big end bearing. That is normally lubricated by oil pressure generated primarily by centrifugal force within the crank. Low rpm = very little oil.
It is an aero engine designed to be started, warmed up at 1000rpm and then flown at a continuous 2000+ rpm. Drive it flat out but don't idle it for longer than you need to get it into gear.

David
Reply With Quote
  #6  
Old 19-07-18, 05:03
Malcolm Towrie Malcolm Towrie is offline
Registered User
 
Join Date: Aug 2015
Location: Whitby, Ontario, Canada
Posts: 306
Default

Thanks for the great information, guys.

I should clarify. We have two Sherman's, one changes gear ok, this one doesn't, and it has been getting worse. The various guys who have been driving them for years agree on that. So I think it is more than gear changing technique.

But that is interesting what you are saying about short shifting from 2 to 3. Personally, I have being doing the opposite, taking it up to 2000 rpm in 2, pushing the clutch in, selecting N, letting the clutch up, and then waiting a while before dipping the clutch again to select 3. I guess that ends up being the same as short shifting as I'm just waiting in N to let the high rpm come down. I'll try the short shift.

One concern I had about short shifting is that lugging seems to be considered a no-no for screamin' Jimmies, even if they pull like trains at low rpm. Tough on the rod bearings, due to lower oil pressure I believe. Anyway we're putting maybe 30 miles a year on the old girls so we unlikely to wear them out.

By the way, the gearbox was originally synchromesh, but it sure isn't now. We have to double clutch up and down on this tank. Just worn out, I guess.

I don't know what oil is in the gearbox, or how old it is. I am just getting involved in these tanks. We should replace it with fresh SAE 50 or maybe 15w40 to see if that helps.

The engine with the slow dropping rpm doesn't seem to be burning oil, but that's a good point about oil burning keeping the rpm up. I'll see if this engine has a history of needing oil top-ups.

We have synchronized both engines by holding them at 1000, 1500, and 1800 rpm when disconnected (after confirming the tachs were accurate) but that has not resolved the difference in rpm drop between them.

We have almost confirmed both clutches fully disengage. When running each engine on its own with the clutch disengaged, the prop shaft could be turned by hand.

Malcolm
Reply With Quote
  #7  
Old 19-07-18, 08:26
Maurice Donckers Maurice Donckers is offline
Registered User
 
Join Date: Feb 2004
Location: Beek Holland
Posts: 403
Default

another warning about changing oil , don`t put modern class 5 oil in a synchro box , this oil is not good for brass parts . use straight 50 in this case .
Reply With Quote
  #8  
Old 19-07-18, 10:57
David Herbert David Herbert is offline
Registered User
 
Join Date: Mar 2010
Location: Ayrshire, Scotland - previously Suffolk
Posts: 547
Default

As Maurice said, any oil with EP additives will attack brass and bronze parts very aggresively. Almost any WW2 gearbox has bronze selectors so synchro or not, do not use EP / class 5 oils at all.

A Sherman gearbox shares its oil with the controlled differential steering system and the final drives. If you put EP into the final drives (as used in most modern tracked vehicles) it will migrate to the steering and main gearbox so no steering, brakes or synchro !

David
Reply With Quote
  #9  
Old 19-07-18, 12:22
Kirk Armitage's Avatar
Kirk Armitage Kirk Armitage is offline
Registered User
 
Join Date: Sep 2003
Location: Ottawa valley
Posts: 98
Default

If both those 6 L's by themselves, linkage disconnected from each other, idle down properly, I would have a hard look at the connecting linkages for play,binding when the motors are torquing etc. Those two rack are literally one when connected together, affecting both motors. If one motor seems slow to come down , disconnected by itself, I would look at ,a sticky injector, hi - low spring pack in the governor is sticking from condensation, and or a buffer screw in to far ( if those early govenors had them)
Love those old Detroit's. K Armitage
Reply With Quote
  #10  
Old 20-07-18, 03:52
Malcolm Towrie Malcolm Towrie is offline
Registered User
 
Join Date: Aug 2015
Location: Whitby, Ontario, Canada
Posts: 306
Default

Thanks for the warnings about the modern GL-5 EP gear oils. Since the original manuals spec SAE 50 oil, I suspect that is what has always been used, at least here at the museum. Mind you, the brake steering isn't anything to write home about either, especially in first gear.

Kirk, I don't think we checked each engine idling down after hand operating the speed control lever with the linkage disconnected. I'll check that. The low speed and high speed springs look in good shape with good idle speed control and max rpm limited to about 2200 rpm. The buffer screw each side is just snugged up enough to tame the idle hunt so I don't think it's screwed in too far.

Malcolm
Reply With Quote
  #11  
Old 22-07-18, 06:27
Malcolm Towrie Malcolm Towrie is offline
Registered User
 
Join Date: Aug 2015
Location: Whitby, Ontario, Canada
Posts: 306
Default

Today, we tested the slow responding engine with the governor completely disconnected from the rack control tube rod, so just hand operating the rack tube. Throttle response was great, very fast. So that seems to rule out oil burning or sticky injector.
Then we tested response of both engines with the linkage disconnected from both governors. So hand operation of the speed control lever on the governor of each engine. Again it was good for both engines. Raising rpm to 1500 and snapping the lever back to idle showed both engines idling down fast.

Connecting the linkages showed problems. Despite syncing both engines earlier, they're were now badly out of sync, with one revving to 1500 and one barely coming off idle. I found one clevis pin badly loose in the clevis, and the bushings supporting both ends of the throttle cross shaft that connects the two engines badly worn. So as Kirk said, that may be the culprit. We'll fix the looseness and go from there.

Thanks again.

Malcolm
Reply With Quote
  #12  
Old 22-07-18, 10:58
David Herbert David Herbert is offline
Registered User
 
Join Date: Mar 2010
Location: Ayrshire, Scotland - previously Suffolk
Posts: 547
Default

Well done, you are getting there. This has defeated many people but as you are demonstrating it is fixable.

David
Reply With Quote
  #13  
Old 08-08-18, 21:23
45jim 45jim is offline
Registered User
 
Join Date: May 2010
Location: Woodstock, ON
Posts: 154
Default Hand throttle control

I wonder which A2 that is? I hope its Bart.

You have already established the engines are fine separately so follow the instructions on page 258 of the manual which cover the throttle linkage adjustment and engine synchronising.

Basically, disconnect the throttle linkage and then progressively reconnect the linkage from rear to front checking each section as you go to isolate the problem area.

The linkage bearings in the governor case are also a known problem area in the 6-71. The lever often moves easily when not connected to the rest of the linkage, but when connected up the side load binds the bearing. If its not Bart, we had changed out the governor covers and bearings to cure a similar problem. But that was a long time ago.
Reply With Quote
  #14  
Old 10-08-18, 17:09
jdmcm's Avatar
jdmcm jdmcm is offline
Registered User
 
Join Date: Dec 2010
Location: Surrey, BC, CDN
Posts: 672
Default

Poor steering in the Sherman that gets worse over time can be the fault of poorly adjusted brakes or a poor operator. If the brakes are dragging they will get hot and end up glazing the pads and brake drums, if someone likes to keep pressure on the levers or make long turns with the lever applied instead of shorter chopping motions the drums will get very hot, I have even seen them crack. Drums can be removed and resurfaced but pads will need to be replaced.
Reply With Quote
  #15  
Old 16-08-18, 04:59
Malcolm Towrie Malcolm Towrie is offline
Registered User
 
Join Date: Aug 2015
Location: Whitby, Ontario, Canada
Posts: 306
Default

Quote:
Originally Posted by 45jim View Post
I wonder which A2 that is? I hope its Bart.

You have already established the engines are fine separately so follow the instructions on page 258 of the manual which cover the throttle linkage adjustment and engine synchronising.

Basically, disconnect the throttle linkage and then progressively reconnect the linkage from rear to front checking each section as you go to isolate the problem area.

The linkage bearings in the governor case are also a known problem area in the 6-71. The lever often moves easily when not connected to the rest of the linkage, but when connected up the side load binds the bearing. If its not Bart, we had changed out the governor covers and bearings to cure a similar problem. But that was a long time ago.
Yes, it's Bart that is having these problems. Thanks for the info about the linkage bearings in the cover. I had the right hand governor apart, which is the side acting up the most and the cover bearings felt snug and free, so we'll focus on the obvious linkage problems but keep that in mind.
Malcolm
Reply With Quote
  #16  
Old 17-08-18, 19:28
45jim 45jim is offline
Registered User
 
Join Date: May 2010
Location: Woodstock, ON
Posts: 154
Default Tank driving

At no time should a driver of any tracked vehicle (with a controlled differential) make "choppy" movements of the tiller bars to steer the vehicle. Smooth applications of the required force that result in gradual loading of the brakes are the most efficient. Sharper turns require more force and consequently more throttle - but smoothly.

The brakes shoes never apply perfectly evenly around the drum surface as they are attached to each other by pins that wear that allow from some out of phase movement. Gradual application allows the shoes to settle on to the drum and spreads the braking action over the whole braking surface greatly reducing wear and tear. Quick pulls on the tiller bars can cause overheating in localized areas of the shoe surface and induce glazing.

Having taught many tracked drivers from M113 to Leopards over the years the ones that drive smoothly with care and attention to the vehicle rarely are the ones buying cases of beer for RCEME recovery crews. The best loved drivers of wrench benders were the ones that understood their vehicle and drove it with due care, not like something they stole.

When we restored our Sherman and Centurion in Calgary the old hands came out to give us some pointers and they overwhelmingly agreed that driving them firmly but smoothly was the key to long life. No different than a modern tank.

One of the problems for most private owners of tracked vehicles is that they rarely get a wide open area to drive the vehicle at high speed (relatively) so are not that familiar with its driving characteristics. Tip toeing around a display area at minimum throttle trying to preserve rubber track shoes does not teach one how to drive a tank!

Your brakes will last much longer if you are firm but smooth in their use.
Reply With Quote
  #17  
Old 17-08-18, 20:01
rob love rob love is offline
carrier mech
 
Join Date: Feb 2003
Location: Shilo MB, the armpit of Canada
Posts: 7,517
Default

Agree with the "non-choppy" use of brakes, with the provisio that you do not drag a tiller bar for long periods either. For example, compensating for road crown.

During the long 100 mile drives from Winnipeg to Shilo with the M113 FOV, you would almost wear out one arm continually making corrections. You would pull the tiller on and over correct for the wander, then let the tiller bar off. As the vehicle would wander back to the opposite away again, you repeated the process....again and again and again. But to simply pull the tiller bar to the point of allowing the drag compensate for the wander and then leaving it there would just result in overheating and burning the differential oil.

Perhaps the term "a series of smooth applications" might have been what John was trying to say. Simple turns can of course be accomplished by single applications of a tiller bar, as long as the oil temp is kept in it's operating range. When it exceeds that limit is when damage to the shoes and drums is going to occur.
Reply With Quote
  #18  
Old 18-08-18, 01:50
jdmcm's Avatar
jdmcm jdmcm is offline
Registered User
 
Join Date: Dec 2010
Location: Surrey, BC, CDN
Posts: 672
Default

I instantly regretted using the word "choppy". I speak for my own experience here and what if any information I have found that may be helpful to others. What I was driving at is brakes on then brakes off...not constantly holding pressure especially when not in a turn, firm on and then completely off, sometimes requiring levers manually being pushed forward into the off position, that is probably more a result of driving old junk versus a "new" Sherman. And it is very easy...and tempting to adjust brakes too tight so they have very quick response on the levers...especially cold and in a parking lot maneuvering around other vehicles looking for your "pimp display parking spot." (and a few gallons of soapy water will help you save that rubber track, especially on concrete!)

IMHO I disagree about holding brakes on for extended periods of time...I have no experience in M113, Leo etc, but the vehicles I do have, Sherman's among them, that friction creates heat...heat makes drums expand and crack, shoes glaze and oil burn

From my own experience on 40 acres in all gears...the brakes begin to work better once warmed up...especially the earlier single anchor brakes...the later twin anchors feel like a sports car in comparison. Also pad material can be a consideration...I happened on a quantity of Israeli re-lined brake shoes...damn near knock your teeth out when you pull the sticks!

Reminds me of the scene from Magnum Force where Dirty Harry is pretending to be the Captain of a jetliner to thwart a hijacking...after a couple of minutes and a few odd maneuvers the co-pilot turns to Harry and says "excuse me Captain, this may sound funny...but can you fly? In classic Harry fashion...Eastwood replies..."Nope...never had a lesson"
Reply With Quote
  #19  
Old 19-08-18, 05:03
Malcolm Towrie Malcolm Towrie is offline
Registered User
 
Join Date: Aug 2015
Location: Whitby, Ontario, Canada
Posts: 306
Default

I'm a bit confused by the last few posts on steering. I have found that on-off application of the steering levers on our Shermans is more effective for turning on our soft deep dirt arena than steady application. Seems to me firm intermittent application of a steering brake is not a recipe for overheating of the bands and drum as little time is spent in the slipping mode. I can see that half-hearted application of the brake with band slipping certainly is, as Rob says.

Mind you, a factor here is that both our Shermans were down on power due to poor governor and throttle linkage set-up. Since we have significantly improved this problem on one Sherman, Bart, it has much more power and I'm told it will now happily steer around our heavy clay arena with steady steering brake application in 3rd gear!

Also the advice above to "short-shift" from 2-3 has dramatically improved shifting. Thanks!

Malcolm
Reply With Quote
  #20  
Old 22-08-18, 23:21
45jim 45jim is offline
Registered User
 
Join Date: May 2010
Location: Woodstock, ON
Posts: 154
Default More on Steering...

Take it from people who had to keep a fleet of vehicles operational in the field that firm and smooth applications of the steering brakes is the only way to drive. You might think the Army is awash with cash and drivers were free to beat vehicles into submission without any regard to the damage they did, but that is a totally incorrect conclusion. Good crews looked after their vehicles and those that didn't often found significant displeasure upon meeting the SSM and Maint WO to explain the damage. Incorrect answers often resulted in an escorted quick march on the CO's carpet.


This idea that correct use of the vehicle brakes will cause damage and "that friction creates heat...heat makes drums expand and crack, shoes glaze and oil burn" is frankly hogwash. The brakes on the vehicle (tanks especially) are designed to stop a fully loaded, fully fuelled and fully crewed tank from top speed to zero in a known and very short distance. The tank must be capable of doing this over and over without overheating the brakes. Tactical necessity require the brakes to be especially resilient when they are responsible for both steering and stopping. Adequate cooling in the Sherman differential is provided by about 40 gallons of oil!

The fact that the Sherman (like the M113) use a controlled differential where power from the braked track is applied to un-braked track requires slipping of the braked shoes over the drum for a smooth and controlled turn to occur. This idea that short applications of full or even partial brake followed by no brake is somehow easier on the brake linings is ridiculous. This jerky action creates stresses in the whole drive line that are just not necessary.

One thing the Sherman tank does not have is pivot steer. If you look under the hood of a M113 you will see a pair of disc brakes on either side of the differential. These are for completely stopping one track and applying all the residual energy to the other, this results in the sharpest possible turn without neutral steer. They even have separate levers for use, they are not for normal steering (similar to a T16). Your suggested driving actions of jerking the tiller bars to facilitate sharp turns appears to be an attempt to emulate that missing low-speed steering option.

The major teaching point for new tank drivers in moving a tracked vehicle at slow speeds in a confined area was to increase the throttle to give the differential some torque to work with. If you are trying to turn and the tank stops or slows too much you need to increase the throttle to complete the turn with the brake applied, not release the brake. A driver must be in control of the tank, not a passenger.

An interesting WW2 film on tank driving:

https://www.youtube.com/watch?v=73Lxw5OQDmc
Reply With Quote
  #21  
Old 23-08-18, 04:13
jdmcm's Avatar
jdmcm jdmcm is offline
Registered User
 
Join Date: Dec 2010
Location: Surrey, BC, CDN
Posts: 672
Default

Been a tough week, first I found out from Rudy Guiliani that "truth isn't truth" and now I find the rules of thermodynamics no longer hold...friction no longer makes heat! Can't overheat a Sherman differential even if you ride the brakes like Lionel Ritchie...ALL NIGHT LONG! I think I will keep on with my dilettante understanding of tank driving and stay off the brakes as much as possible...hogwash it may be, but I aint blown up a Sherman diff yet...

Last edited by jdmcm; 23-08-18 at 07:48.
Reply With Quote
  #22  
Old 23-08-18, 22:16
David Herbert David Herbert is offline
Registered User
 
Join Date: Mar 2010
Location: Ayrshire, Scotland - previously Suffolk
Posts: 547
Default

I just came across this which I think is quite clear :

Click image for larger version

Name:	m113 turn.jpg
Views:	6
Size:	294.8 KB
ID:	101721

Click image for larger version

Name:	m113 turn 2.jpg
Views:	8
Size:	474.8 KB
ID:	101722

The British army teaches drivers of tracked vehicles with mechanical steering brakes, which is everything up to and including Chieftain, but not vehicles with hydrostatic steering like Warrior or Leopard, to make SHORT full brake applications, not long gentle ones which WILL overheat brake disks or drums, whether submerged in oil or not. The brake surface cannot be cooled by the oil if held on for a long period as it has been squeezed out, and the result will be premature wear on the linings and overheated disks or drums which will then either grab or break up with serious results. There are many instances of M113s and FV432s that have suddenly turned 45 degrees and shot off the road into the bushes (or buildings) because the driver has been trying to correct for drift by holding a brake on for long periods.

David
Reply With Quote
  #23  
Old 24-08-18, 03:52
Malcolm Towrie Malcolm Towrie is offline
Registered User
 
Join Date: Aug 2015
Location: Whitby, Ontario, Canada
Posts: 306
Default

Quote:
Originally Posted by 45jim View Post

The fact that the Sherman (like the M113) use a controlled differential where power from the braked track is applied to un-braked track requires slipping of the braked shoes over the drum for a smooth and controlled turn to occur. This idea that short applications of full or even partial brake followed by no brake is somehow easier on the brake linings is ridiculous. This jerky action creates stresses in the whole drive line that are just not necessary.

Im afraid I'm going to offend you here but I wonder if you understand how the "gear steering" in the Sherman or the M113 work? The brake being applied by each steering lever acts on a planetary gear set dedicated to driving the track on that side. It locks one of the three elements in the planetary gear set (the sun gear if I recall correctly) which reduces the gear ratio to that side and slows down the track. The differential, by nature of its design, speeds up the track on the other side and the turn happens. There is normally no need for brake slippage in this action. In fact, firm application of the steering lever to prevent slipping and overheating is best as Dave points out in his post.

Malcolm
Reply With Quote
  #24  
Old 27-08-18, 19:13
45jim 45jim is offline
Registered User
 
Join Date: May 2010
Location: Woodstock, ON
Posts: 154
Default "Brake Slippage"

Malcolm, don't worry about offending I'm not a millennial!

I understand the system well and to be clear you must separate braking action and differential action to understand for it is not gears that steer the vehicle, its the drivers manipulation of the steering brakes and the resultant action of the differential which causes a change in direction.

The differential located in a tank behaves in a similar manner to one in the rear of a car except while the one in the car responds to friction generated between the tires and the road the controlled differential responds to driver induced braking action on the steering brakes.

The very reason brake bands are used in this application is because the drum must be slowed to a stop. The brake bands exert friction on the drum surface causing it to slow and then stop. This means that for a period of time the bands are slipping over the surface of the drum providing increasing amounts of friction until the drum is stopped. Just like the brake on a car, it is braking not "locking".

And if you notice on the Sherman the tiller bars (steering levers) have a significant range of motion. We all know from driving tracked vehicles that the turn starts after the free play is taken up (when the brakes first touch the drum) and gradually as we continue to pull back on the bar the turn sharpens. So, from the point that the free play is taken up until the bar is pulled back as far as possible the brake bands are in contact with the drum.

If one were to continue with minimal pressure on the lever a very gentle turn indeed would be produced and at no time would the drum stop and the brake bands would continue to slip, eventually glaze up and the heat could damage the drum. I believe this is Rob was speaking about. Correcting for the road crown with continuous "left stick" is to be avoided. This is I am sure where jdmcm's concern comes from.

If the tiller bars fully to the rear represents "full on" then every position forward (minus the free play) is one where the brake bands are in contact with the drum but with less pressure (and less braking action). Just like the foot brake in a car the lever motion is directly related to the amount of braking.

The amount of braking (which is variable) is directly related to how much torque is split between the output shafts. This means the torque split is infinitely variable starting from when the brakes start slowing the drum and increasing until the drum is fully stopped. Lots of brake band slipping.



As to how the British Army teaches their drivers I think my comments of:

At no time should a driver of any tracked vehicle (with a controlled differential) make "choppy" movements of the tiller bars to steer the vehicle. Smooth applications of the required force that result in gradual loading of the brakes are the most efficient. Sharper turns require more force and consequently more throttle - but smoothly.

firm and smooth applications of the steering brakes is the only way to drive

is pretty close to:

"To make a turn do it with several short smooth pulls instead of one long pull or a series of quick jerks"

I might not to be to eager to agree with the "several" aspect but its like picking fly shit out of pepper. Perhaps Rob got it just right with "a series of smooth applications"

Pretty sure I didn't break any of the laws of thermodynamics in my comments. The Sherman's friction brakes do more than just steer and when they wear out they are replaced just like any other wearing part. I have only seen the diff damaged by the lack of oil, and that was failure of a gear set not the brake linings or drum. They are after all, friction brakes!
Reply With Quote
  #25  
Old 28-08-18, 05:31
Malcolm Towrie Malcolm Towrie is offline
Registered User
 
Join Date: Aug 2015
Location: Whitby, Ontario, Canada
Posts: 306
Default

Jim,
Interesting discussion!
We may be quibbling over terminology here, but it is gears that steer the Sherman. The slowing down that happens to one side of the ingenious Cletrac diff by pulling back a tiller is caused by stopping (or as you suggest, slowing) the large steering/brake drum. This reduces the drive gear ratio to that track. The differential then does its thing and speeds up the other track.

On the subject of just slowing down the steering/brake drum, I get the impression the jury is against this type of operation, which makes sense to me. Braking of the drum should be firm and positive to minimize slippage and heat. This means the radius of a turn is dictated by the number of "short, smooth pulls" that are performed during a turn. Obviously, the tightest turn is when the brake is held firmly on throughout the turn.

It's not clear to me in your last post whether you are advocating light, slipping application of the brake for a large turn radius, or not?

By the way, when I first drove a Sherman, I thought there was something wrong with the steering. I since found out that a controlled diff gives a fixed turn radius regardless of speed, and the Sherman's turn radius (or was it circumference?) is around 70'! No wonder she's a pain to manoeuvre in tight spaces.

Malcolm
Reply With Quote
  #26  
Old 28-08-18, 06:19
Malcolm Towrie Malcolm Towrie is offline
Registered User
 
Join Date: Aug 2015
Location: Whitby, Ontario, Canada
Posts: 306
Default

I found this good cross section of the right hand Sherman diff and final drive. Colour coding the separate rotating parts helps me understand this stuff.

The orange part is the diff carrier which is driven round by the pinion gear shown on the gearbox output shaft.

When driving straight ahead, the three yellow planet gear shafts supported in the orange carrier rotate at the same speed as the carrier obviously. The small gears at the left of the yellow shafts are interlocked with matching small gears on the three planet gear shafts driving the left hand final drive (not shown). The left hand small gears want to turn the opposite way, but they can't during straight driving, so none of the yellow shafts rotate relative to the carrier.

So the locked yellow large gears freewheel the green assembly, which includes the brake drum, at the same rpm as the orange carrier.

The locked smaller gears on the yellow shafts also drive the large pink gear at the same speed as the orange carrier, which drives the track through the pink final drive.

When the right tiller is pulled back, the band locks around the large drum and the green assembly is stopped.

This forces the yellow shafts to rotate relative to the orange carrier as they "walk" around the locked green gear.

The smaller yellow gears driving the large pink gear also "walk" around the the pink gear, which drives it at a lower speed, giving the gear reduction to the right final drive.

As mentioned above, the small yellow gears are also engaged with the equivalent yellow shafts for the left hand drive. The rotation of the small right hand yellow gears during a turn speeds up the rotation of the left hand yellow gear, which increases the speed of the left hand track.

How mere humans come up with this stuff boggles my mind.

Malcolm

Click image for larger version

Name:	Sherman controlled diff in colour (smaller).jpg
Views:	8
Size:	1.16 MB
ID:	101836

Last edited by Malcolm Towrie; 28-08-18 at 06:32.
Reply With Quote
  #27  
Old 28-08-18, 08:41
Lynn Eades Lynn Eades is online now
Bluebell
 
Join Date: May 2003
Location: Tauranga, New Zealand
Posts: 5,534
Default

From a position of no experience of Shermans, save standing on the "Fury" tank, at Bovington, here goes:
Essentially Malcolm, the function is no different from a U.C. drive axle The yellow gears are the spider gears of the diff. When your tank is travelling straight ahead, they are static in the diff carrier. When the Sherman is turned by the application of one brake, the opposite side speeds up by the amount the braked side slows down. (inversely proportional) If you stop one side the rotation speed of the other side doubles. (this does not take into account the extra load and horse power to achieve this)
Anyhow this affects the turn. This diff functions like your average car. Yes, it has extras,(brakes, the hub reduction, size, etc)but the function is the same.

How many times you turn, how tight, What the surface is and probably a whole lot of other things will affect heating of the brakes.
How does a brake work? It converts kinetic energy (movement) into heat. That's it.
Like I said, no Knowledge. I was surprised to find that it never had steering clutches. I guess there's a good reason. I just haven't figured it out yet.
__________________
Bluebell

Carrier Armoured O.P. No1 Mk3 W. T84991
Carrier Bren No2.Mk.I. NewZealand Railways. NZR.6.
Dodge WC55. 37mm Gun Motor Carriage M6
Jeep Mb #135668
So many questions....
Reply With Quote
  #28  
Old 28-08-18, 12:20
David Herbert David Herbert is offline
Registered User
 
Join Date: Mar 2010
Location: Ayrshire, Scotland - previously Suffolk
Posts: 547
Default

I am sorry Lynn but no, it does not function like the braked differential steering in a universal carrier.

If you make a full brake application of a Controlled Differential (Cletrac) system, and fully lock the brake drum from turning (no slipping), the output (pink) shaft on the braked side will rotate at about half the speed of the corresponding shaft on the unbraked side. This is caused by the difference in the gear ratios between the green/yellow and the pink/yellow gear sets and will be designed as a compromise between the very large radius turns needed at high road speeds and the small radius turns needed when manoevering. On M113 family and T16 carriers they gave themselves a bit more scope for having a large minimum turn radius on the controlled differential by having brakes on the output shafts for pivot turns as well. These ARE directly equivilent to the UC setup.

Thank you Malcolm for posting the nicely coloured in drawing, it makes this discussion much easier.

Lynn, the reason that modern tanks (anything after Valentine) do not have steering clutches / brakes like crawler tractors is that at anything over about 10 mph clutch and brake steering becomes very difficult to control, particularly downhill. In fact on a steep downhill the tank can turn the opposite way to that intended as soon as you release the steering clutch. That can be quite hard to predict and is quite exciting. Also with clutch and brake steering the net speed of the vehicle is reduced when steering which increases the likelyhood of the inside track just skidding and no turn resulting - I have done this with a Valentine on a road and was lucky not to cause any damage. A Sherman will go sideways if you steer too hard but at least it turns.

David
Reply With Quote
  #29  
Old 28-08-18, 18:25
45jim 45jim is offline
Registered User
 
Join Date: May 2010
Location: Woodstock, ON
Posts: 154
Default And more Sherman steering discussion....

I am not an advocate of continuous light pressure on the tiller bars. Smooth but firm applications when needed. And indeed they aren't the most manoeuvrable at slow speed in tight spaces. The M113 was equipped with pivot steer for that very reason!

I found this video supposedly of a weasel controlled diff in action. Nice animation and for arguments sake mostly correct. The other one is listed as braked differential steering, less clear but neat nonetheless.

https://www.youtube.com/watch?v=gxfeXAi-C7k

https://www.youtube.com/watch?v=SqBQRS3J0yQ4
Reply With Quote
Reply

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
Wanted: GM Diesel Division brochure Robin Craig For Sale Or Wanted 3 22-01-14 10:25
Lister Diesel Mike Kelly WW2 Military History & Equipment 0 18-05-12 03:05
CMP Diesel Conversions??? BEN-RIPPINGHAM The Softskin Forum 21 16-04-08 16:12
Sherman M4A2 diesel engine designation Larry Hayward The Armour Forum 18 11-02-05 15:33
Diesel Repower JD Baillie Post-war Military Vehicles 3 02-07-04 06:25


All times are GMT +2. The time now is 22:58.


Powered by vBulletin® Version 3.8.4
Copyright ©2000 - 2024, Jelsoft Enterprises Ltd.
Copyright © Maple Leaf Up, 2003-2016