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 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

[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