Sherman A4A2 diesel 2-3 shift problem

[Malcolm Towrie]

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

[tankbarrell]

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.

[David Herbert]

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

[Jesse Browning]

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.

[David Herbert]

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

[Malcolm Towrie]

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

[jdmcm]

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...

[David Herbert]

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

m113 turn.jpg

m113 turn 2.jpg

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