Sherman A4A2 diesel 2-3 shift problem

[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

[Maurice Donckers]

another warning about changing oil , don`t put modern class 5 oil in a synchro box , this oil is not good for brass parts . use straight 50 in this case .

[David Herbert]

As Maurice said, any oil with EP additives will attack brass and bronze parts very aggresively. Almost any WW2 gearbox has bronze selectors so synchro or not, do not use EP / class 5 oils at all.

A Sherman gearbox shares its oil with the controlled differential steering system and the final drives. If you put EP into the final drives (as used in most modern tracked vehicles) it will migrate to the steering and main gearbox so no steering, brakes or synchro !

David

[Kirk Armitage]

If both those 6 L's by themselves, linkage disconnected from each other, idle down properly, I would have a hard look at the connecting linkages for play,binding when the motors are torquing etc. Those two rack are literally one when connected together, affecting both motors. If one motor seems slow to come down , disconnected by itself, I would look at ,a sticky injector, hi - low spring pack in the governor is sticking from condensation, and or a buffer screw in to far ( if those early govenors had them)
Love those old Detroit's. K Armitage

[Malcolm Towrie]

Thanks for the warnings about the modern GL-5 EP gear oils. Since the original manuals spec SAE 50 oil, I suspect that is what has always been used, at least here at the museum. Mind you, the brake steering isn't anything to write home about either, especially in first gear.

Kirk, I don't think we checked each engine idling down after hand operating the speed control lever with the linkage disconnected. I'll check that. The low speed and high speed springs look in good shape with good idle speed control and max rpm limited to about 2200 rpm. The buffer screw each side is just snugged up enough to tame the idle hunt so I don't think it's screwed in too far.

Malcolm

[Malcolm Towrie]

Today, we tested the slow responding engine with the governor completely disconnected from the rack control tube rod, so just hand operating the rack tube. Throttle response was great, very fast. So that seems to rule out oil burning or sticky injector.
Then we tested response of both engines with the linkage disconnected from both governors. So hand operation of the speed control lever on the governor of each engine. Again it was good for both engines. Raising rpm to 1500 and snapping the lever back to idle showed both engines idling down fast.

Connecting the linkages showed problems. Despite syncing both engines earlier, they're were now badly out of sync, with one revving to 1500 and one barely coming off idle. I found one clevis pin badly loose in the clevis, and the bushings supporting both ends of the throttle cross shaft that connects the two engines badly worn. So as Kirk said, that may be the culprit. We'll fix the looseness and go from there.

Thanks again.

Malcolm