Sherman A4A2 diesel 2-3 shift problem

[Malcolm Towrie]

Originally Posted by 45jim

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

[45jim]

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!

[Malcolm Towrie]

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

[Malcolm Towrie]

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

Sherman controlled diff in colour (smaller).jpg

[Lynn Eades]

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.

[David Herbert]

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

[45jim]

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