Sherman A4A2 diesel 2-3 shift problem

[rob love]

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.

[jdmcm]

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"

[Malcolm Towrie]

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

[45jim]

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

[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