Hetzer preselect gearbox compared to Ferret

[Malcolm Towrie]

This is quite specific and I dont expect a flood of responses , but we have just acquired a nice restored Jagdpanzer 38(t), aka Hetzer, thanks to a rather wealthy benefactor.
It has a Wilson preselect gearbox, like a Ferret, or more correctly, a Praga-Wilson preselect gearbox. It has the same left hand pedal, similar to the gear change pedal on a Ferret. But what interests me is the gearbox is bolted directly up to the differential with no evidence of a fluid coupling like a Ferret, or a clutch.

It seems that, unlike the Ferret, the gear change pedal has two functions as it rises from the hull floor, first it engages the gear selected, and then clamps the band around the drum. This clamping serves as the clutch engagement. I say this because when you come to a stop, you have to depress the pedal and select neutral to prevent the engine stalling, unlike a Ferret.

Anyone any insight? Manuals for WWII German vehicles are damn near unobtainable.

Another thing I thought was interesting: the steering and brake drums are fully exposed. You can see the steering band clamp around a steering drum when you pull back on one of the (horizontal) tiller bars, and you can see the drum stop, confirming you are getting the designed steering radius for whatever gear you are in. The trouble is the steering radius is horrible, even in first gear. It's an extremely unmaneuverable vehicle. Much worse than a Sherman, and they're not great. And no neutral turn. So to aim that gun, you had to drive forwards or backwards. I can see the desirability of setting it up to fire down a bottleneck for oncoming tanks to keep you in the narrow 16 degree traverse range of the gun.

Malcolm

[Richard Farrant]

Hi Malcolm
When the Wilson box was fitted in the prewar ERA racing cars there was no fluid flywheel or clutch.

[David Herbert]

Hi Malcolm,
If there was a fluid flywheel or clutch, it would be between the engine and the Wilson gearbox, not between the gearbox and the steering system.

Also the selection and engagement of gears in any Wilson box is done by engaging the band brake for that gear. Thus the selection and the clutch is combined into one action as there is effectively one clutch (the band brake) for each gear. When you press the pedal down fully, any brake band that was engaged is released (which is why you need to press it all the way down) and on raising the pedal the next selected gear is engaged. Only one new gear can be selected at a time as there is a hinged sprag for each brake band that only engages if pushed out by a cam. All the cams are on a shaft that is turned by the gear selection lever and only one sprag is pushed out at a time. So the CHOICE of gear is controlled by the gear selector lever and the disengagement / engagement by the gear change pedal.

I have no experience of Pz38t but I thought that the steering was similar to the system on British Crusader, ie a bevel box (no differential) to turn the drive 90 degrees and then epicyclic / band brake steering units inboard of each final drive to give: High, low and stop separately for each track ( or at least drive and stop). There must be brakes that work together for stopping the vehicle and on a small tracked vehicle designed in the 30s it would be very unusual for it not to be possible to engage them separately to give a pivot steer.

I expect to be corrected about the steering system so if anyone has a nice drawing of the system used in a Hetzer it would be great to see it.

David

[maple_leaf_eh]

The first source that comes to mind is the prolific writer Steve Zaloga. If he doesn't have the information, he probably knows who does. No, I don't have his contacts.

The man is a professional writer, so don't expect him to cough up research for free. Be ready to pay costs.

[James P]

I can only suggest that you people send a couple reps to the next Thunder Over Michigan airshow and connect with G13/38t owners to become better acquainted with the vehicle you now have and the finer points of maintaining and operating it in a safe manner. Take notes, take pics, ask questions, become educated .

[Jordan Baker]

Did it not come with G13 manual? I would have thought for that kind of money it would

[Malcolm Towrie]

Originally Posted by Richard Farrant

Hi Malcolm
When the Wilson box was fitted in the prewar ERA racing cars there was no fluid flywheel or clutch.

Richard, I'd heard the were used in race cars. I suppose the limited need for a clutch in a race car allowed the use of the bands as clutches.

I think the Wilson box must have been perfect for those old race cars. Preselecting the gear for the next corner would allow you to keep both hands on that huge steering wheel as you wrestled it into and around.

Malcolm

[Malcolm Towrie]

Originally Posted by David Herbert

Hi Malcolm,
If there was a fluid flywheel or clutch, it would be between the engine and the Wilson gearbox, not between the gearbox and the steering system.

Also the selection and engagement of gears in any Wilson box is done by engaging the band brake for that gear. Thus the selection and the clutch is combined into one action as there is effectively one clutch (the band brake) for each gear. When you press the pedal down fully, any brake band that was engaged is released (which is why you need to press it all the way down) and on raising the pedal the next selected gear is engaged. Only one new gear can be selected at a time as there is a hinged sprag for each brake band that only engages if pushed out by a cam. All the cams are on a shaft that is turned by the gear selection lever and only one sprag is pushed out at a time. So the CHOICE of gear is controlled by the gear selector lever and the disengagement / engagement by the gear change pedal.

I have no experience of Pz38t but I thought that the steering was similar to the system on British Crusader, ie a bevel box (no differential) to turn the drive 90 degrees and then epicyclic / band brake steering units inboard of each final drive to give: High, low and stop separately for each track ( or at least drive and stop). There must be brakes that work together for stopping the vehicle and on a small tracked vehicle designed in the 30s it would be very unusual for it not to be possible to engage them separately to give a pivot steer.

I expect to be corrected about the steering system so if anyone has a nice drawing of the system used in a Hetzer it would be great to see it.

David

David, I messed up, didn't I. I can only attribute it to the lateness of the hour.

Yes, I'm familiar with how a Wilson preselect works (and a brilliant design it is), I just didn't word that second paragraph very well. I meant the Ferret doesn't need to use the bands as a clutch, in fact the manual warns against it, IIRC. So the gear selected for pulling away is selected, the gear change pedal is pushed down, the selected gear engages, the pedal is let up quickly so the band bites quickly, avoiding slippage, and the fluid coupling prevents the engine from stalling.

But the Hetzer appears to use the band as a clutch when pulling away from a stop. As I said, you have to select neutral (actually "0" on the plate) before letting up the gear change pedal at a standstill or the engine will stall.

Your description of the Crusader steering is interesting. That may be the design used in the Hetzer as the "differential" casing is surprisingly compact. It being a simple bevel box would explain that.

The Hetzer has brake drums outboard of the steering drums but as far as I can see, there is no way to brake individual drums to allow pivot steering. I'll double check.

Malcolm

[James P]

Have you contacted other 38(t) owners like Saumar, Bovington, Sinshiem, Munster or Bruce Crompton for assistance in how to operate the vehicle ?? You start beating the bushes and do some outreach via the net and no doubt you will connect with folks who can provide advice galore and printed material.

[Malcolm Towrie]

James, I don't look after the Hetzer, I just got called to do a repair on it after it arrived and I was interested in the drivetrain. I believe the owner is awaiting some spares from Poland where it was purchased, and I have suggested that he gets copies of whatever manuals they have. I assume they have manuals since they completely restored it.

Malcolm

[maple_leaf_eh]

Malcolm

I saw the reveal and heard the nearly 45 minute presentation. Very interesting. Now that I realize the history, I think a quick call to the Swiss Embassy in Ottawa asking for an introduction to the Swiss military's historical section or their armour museum might be faster. Swiss are a polite people, and they like to know who they are dealing with.

[Tobias Weibel]

Good day to all of you.

I work in the workshop of the Swiss Military Museum Full-Reuenthal. We are one of the largest military museums in Switzerland.
We have two G13 (petrol and diesel engine)

I can organize or scan a G13 manual.

Tobias

[James P]

Great to see another MLU member (Tobias) stepping up and offering assistance. Seriously if you are plonking down a load of cash for a vehicle you should buy all the manuals possible, it is really not all that difficult. Case in point............

https://www.ebay.com/itm/WW2-German-...-/391441687153

[Malcolm Towrie]

Originally Posted by Tobias Weibel

Good day to all of you.

I work in the workshop of the Swiss Military Museum Full-Reuenthal. We are one of the largest military museums in Switzerland.
We have two G13 (petrol and diesel engine)

I can organize or scan a G13 manual.

Tobias

Tobias, that would be very useful to us! We have no documentation to go with this very nice Hetzer we have just acquired. This is a Praga petrol-engined version.
Malcolm

[Tobias Weibel]

Here is a preview







The rest is in the works

[David Herbert]

Ok,
From the above it can be seen that each side functions separately. Slowing one side does not speed up the other so there is no regeneration unlike a controlled differential (Sherman / Stuart) or Merit Brown (Centurion / Cromwell etc).

The two outer bands are simply brakes to stop that track.
Each side has a clutch which is spring engaged like a car clutch. If engaged with no brakes applied that track is in high gear (normal for straight running).

Clutch disengaged with no brakes gives neutral which if the other side is still in high, will result in a gentle turn towards the side in neutral depending on road conditions. However on a down slope the vehicle will turn the other way which can be exciting.

Clutch disengaged and the inner brake band applied will give low gear to that track resulting in a geared turn (if the other track is in high) of a radius depending on the gear ratios.

It is possible to design the controls so that it is possible to have one track in neutral with the brake applied and the other in low, resulting in a pivot turn with one track locked but it seems from Malcolm's earlier post that that has not been done here. On Crusader it was relatively easy as the brake bands were air operated so all that was needed was a rather ingenious valve block.

The Praga-Wilson main transmission I think is a five forward one reverse unit, very similar to those used in cars and buses in the UK.

I do not know what the units are on the front of the engine.

David

[Richard Farrant]

Originally Posted by David Herbert

I do not know what the units are on the front of the engine.

David

Hi David,
I was intrigued to know what the two items were on the drive end of the engine. Using a german to english translation on Google, it seems one is flywheel (logical) and the other means 'sliding clutch'. No doubt some else knows more on this.

[David Herbert]

Centrifugal clutch possibly ?

Sorry, I could have googled it too but was too lazy !

If it is a centrifugal clutch it might be stuck from lack of use which might explain why it is necessary to engage neutral to avoid stalling.

Also I should have thanked Tobias for joining in.

David

[Richard Farrant]

Originally Posted by David Herbert

Centrifugal clutch possibly ?

Sorry, I could have googled it too but was too lazy !

If it is a centrifugal clutch it might be stuck from lack of use which might explain why it is necessary to engage neutral to avoid stalling.

Also I should have thanked Tobias for joining in.

David

David,
There is another word before before 'sliding clutch', which is Sicherheilts, which translates as Safety, so safety sliding clutch. I think centrifugal is most likely as it does not need a separate control.

Yes, thanks to Tobias for the interesting diagrams and welcome to the forum.

[leo tomaiolo]

this is in response to the question about turning in the 'Hetzer'. i have had the pleasure of driving both the Hetzer and Shermans. i don't have much on the technical information of the wilson gear box but i appears as though it's being covered. the Hetzer in question is owned by the American Heritage Museum. it's quite a good runner and very nimble. i my opinion it's as maneuverable as the Sherman all day. the Hetzer has a 2 stage steering. trying to remember how it goes but i believe as the steering controls are set to high speed turning normally and when engaged the steering has a much tighter 'feel' and turns much more responsive. the Hetzer was originaly from the Littlefield collection. it started life as a G13 and was converted back to war time configuration. it has the 6 cylinder petrol engine and 5 speed gear box. i've included some pictures. if you look at the steering controls you will see a button that gets pressed with your thumb to engage the hi/low steering. a side shot of the transmission and drivers postion. last up, me driving and a shot of my truck with my ford.

[Malcolm Towrie]

Fantastic information, Tobias. Thank you very much.

Here's a video I took of the operation of the steering and brakes before I got this info. I'm sure the above will answer my questions.
Malcolm

https://youtu.be/E8c9TRP9nSA

[Malcolm Towrie]

My interpretation of googles interpretation of the first schematic is the flywheel has an integral safety torque limiter, in other words a slipping clutch to protect the drivetrain, rather than a centrifugal clutch. Quite unusual. Maybe because the Wilson gearbox will engage with a helluva bang if some effort is not made to rev-match to the selected gear.

The thing to the right of the flywheel translates to be a step-up or step-down gearbox.

The clutch operation that puzzled me in the video seems to have the effect of disconnecting the steering drum from the drive shaft during gear steering operation to allow gear reduction, and the combined effect of braking the track and disconnecting the track from the drive shaft during brake steering operation.

Malcolm

[Tobias Weibel]

Hi at all

I'm happy to help.
Unfortunately, my technical English is not so good. That's why I can't argue with you.

The two items were on the drive end of the engine are an flywheel and the safety sliding clutch

safety sliding clutch:


Are the Brake Bands new?
It is very important that the brake bands are adapted (grinding)

[Tobias Weibel]

Here is the description of the steering from the manual.
I think it's better if I write this in German.
I don't know which translation program is good.

****************
Fahrt in gerader Richtung

Kein Lenkhebel wird betätigt.
Alle Bremsbänder sind Lose.
Radius- und Ortslenktrommel sind über die Lamellenkupplung (10) miteinander verbunden.
Da beide Trommeln (8/9) fest über die Kupplung (10) miteinander verbunden sind, sind auch die beiden Sonnenräder (12 und 14) über die Ritzelwelle (2) kraftschlüssig.
Beim Antrieb der Kegelradwelle dreht sich der Satellitenkasten (15) mit der gleichen Drehzahl wie die Welle. Da die Planetenräder (13) verschiedene Zähnezahlen aufweisen, können sich diese Räder nicht um ihre eigene Achse drehen..
Folglich dreht sich die Radius- und Ortslenktrommel wie auch die Ritzelwelle drehzahlgleich mit dem Satellitenkasten.

Radiuslenkung

Der Radiuslenkhebel wird zurückgezogen. Dadurch wird:
1. Die Lamellenkupplung (10) ausgekuppelt.
Das heisst die Radius- (9) von der Ortslenktrommel (8) getrennt.

2 Die Radiuslenktrommel (8) durch das Bremsband abgebremst.
Das heisst die Radiuslenktrommel (9) und das kleine Sonnenrad (12) stehen still.

Wenn sich der Satellitenkasten dreht müssen sich nun die grossen Planetenräder (13) auf dem stillstehenden Sonnenrad (12) der Radiuslenktrommel abwälzen. Die kleinen Planetenräder drehen sich drehzahlgleich mit dem grossen. Die Drehrichtung dieser Räder ist dieselbe wie diejenige des Satellitenkasten. Die kleinen Planetenräder (13) wälzen sich nun auf dem Sonnenrad (14) der Ritzelwelle ab. Infolge der Zähnedifferenz des kleinen Planetenräder (13) zum grossen Sonnenrad (14) muss sich dieses Sonnenrad in der Drehrichtung des Satellitenträgers zu drehen beginnen. Bei einer Umdrehung des Satellitenkastens (15) erhalten wir eine Reduktion der Antriebsdrehzahl der Ritzelwelle von etwa einem Viertel. Durch den Drehzahlunterschied der beiden Antriebsräder fährt der Panzerjäger eine Kurve.

Bei gleichzeitiger Betätigung beider Lenkhebel erhalten wir eine Reduktion der Geschwindigkeit. Das Fahrzeug fährt gerade aus.

Ortslenkung

Hebel wird mit eingedrücktem Druckknopf zurückgezogen.
Dadurch wird:
1. Die Lamellenkupplung (10) ausgekuppelt.
2. Die Ortslenktrommel (8) duch das Bremsband (6) abgebremst.
Die Ritzelwelle (2), die Ortslenktrommel ((8) und das grosse Sonnenrad (14) stehen still.

Bei sich drehendem Satellitenkasten (15)wälzen sich die kleinen Planetenräder (13) auf dem grossen, stillstehenden Sonnenrad (14) ab, die grossen Planetenräder drehen sich drehzahlgleich mit den kleinen, in der gleichen Drehrichtung wie der Satellitenkasten.
Durch die Zähnezahlen bedigt, erhalten wir eine Übersetzung auf die Radiuslenktrommeln (9) in entgegengesetzter Drehrichtung des Satellitenkasten. Die Radiuslenktrommel dreht sich mit etwa 4 - 5-facher Drehzahl des Satellitenkasten. Da das Antriebsrad blockiert ist dreht sich das Fahrzeug um die Mitte auf der Auflagefläche der Raupe.

Um eine Beschädigung der Zahnräder des Umlaufgetriebes und denjenigen des Seitenantrieb vorzubeugen, darf die Ortslenkung nur im 1. Vorwärts- und im Rückwärtsgang betätigt werden.

Press the button on the steering lever only in 1st gear or reverse gear.

***************************************

[Niels V]

Possibly you could also reach out to the https://www.facebook.com/thegraebecollection/ they have some experience with that type of gearbox, in their Swedish based Marder replica.

[James P]

Originally Posted by Niels V

Possibly you could also reach out to the https://www.facebook.com/thegraebecollection/ they have some experience with that type of gearbox, in their Swedish based Marder replica.

Bob and his son are great guys, I would also suggest the Ontario museum people contact David C in Michigan, another Hetzer/G13 owner.

[David Herbert]

Tobias,
Once again thank you for your help here. Your technical English is much better than my German or French so I am working just from the drawings and the important instruction not to press the button in any gear other than R or 1.

Malcolm,
As you say the "safety" clutch is only a torque limiter, though I am puzzled why they didn't use a fluid flywheel which was well established technology by then and would have been much nicer to drive.

To re-write my previous post in the light of the video and more from Tobias:

If no levers are pulled the drive goes to both tracks at exactly the same speed so the vehicle moves in a straight line.

If one lever is pulled all the way without pressing the button, the clutch inside the brake drum is released and the brake band nearest the centre of the vehicle tightened giving a lower ratio drive to that track. That side track then turns slower than the other which remains in high and the vehicle turns with apparently quite a large radius.

If one lever is pulled while pressing the button, the clutch is released as above but the middle brake band is applied which, if the lever is pulled hard, completely stops that track causing a pivot turn. It seems that doing this does not change the other track which therefore remains in high. This type of turn requires a lot of power and also tends to promote a skid on a hard surface rather than a turn, hence the instruction to only do it in 1 or R.

In both of these types of turn there must be a brief neutral (clutch released) before the brakes are applied or there will be excessive brake wear. However, in that neutral position the vehicle will turn the wrong way on a downhill slope as that track is no longer holding it back. This can be very dangerous and is a major reason why clutch and brake steering (which this is a form of) stopped being used as tanks became faster. Also the tendency to just skid in a straight line without actually turning can be quite exciting as I found out with a Valentine once. Adjustment is key here.

In theory, if you pulled one lever while pressing its button and the other without pressing its button, you would get a pivot turn towards the button pressed side, with the other track in low. This would be a bit more relaxed than having the driving track in high but would require a lot more training to get right in a tight spot so I suspect was discouraged.

Controlled differentials are SOoooo much easier !

David

[Malcolm Towrie]

Thanks, David. Good summary.

I also notice from the German instructions that pulling both tillers back with the thumb buttons not depressed results in a straight line gear reduction to whatever gear you are in.

The instructions say the reduction of the drive speed is "of about a quarter" which I assume means to 75% of what the other track is driving at, rather then to 25%.

Our T-54 does this too which is quite handy when your ground guide is looking a bit stressed.

Malcolm

[James P]

I would in the strongest of terms, suggest you operate your Hetzer with the utmost of care, considering it is akin to driving a tank in a permanent state of "closed down" with the driver viewing the world through a letter opening in a door.

[Malcolm Towrie]

Originally Posted by Tobias Weibel

Hi at all

I'm happy to help.
Unfortunately, my technical English is not so good. That's why I can't argue with you.

The two items were on the drive end of the engine are an flywheel and the safety sliding clutch

safety sliding clutch:


Are the Brake Bands new?
It is very important that the brake bands are adapted (grinding)

Tobias, thanks again for the new schematics.
I find it quite humbling that so many Europeans, like yourself, are quite fluent in
English, yet we British and North Americans are hopeless at European languages. Thank God for Google Translate.

I don't know the condition of the brake bands but they do look new. And if you are referring to grinding of the brake drums, no, I dont think they have been ground, they look a bit scored.

Malcolm