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Walter_Sobchak

Transmissions and final drives

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I figured we should have a thread dedicated to transmissions and final drives of AFVs.  I'll kick the thread off by going old school with this article from the May-June 1921 issue of the Ordnance Journal on "Final Drive for Combat Vehicles."

 

final-drive-for-combat-vehicles-page-1.j

final-drive-for-combat-vehicles-page-2.j

final-drive-for-combat-vehicles-page-3.j

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From the first article:

 

-The steering shown in figure 1 is how a Bren gun carrier steered.  That, and it had pivoting road wheels and flexible track.  This isn't a very good steering system for heavier tanks because it wastes too much energy.

 

-The steering shown in figure 2 is how most American WWII tanks steered; including the sherman, stuart, lee, and pershing.  It is also referred to as the "Cletrac" system, as the Cleveland Tractor Company made a lot of vehicles with this steering system.  Some of the German half-track designs (but none of their tanks) used this steering in conjunction with the pivoting front wheels because GERMAN HALF-TRACK DESIGNS WERE INSANE.  A well-designed Cletrac system was probably one of the better steering systems available for a tank in WWII, since it allows a precise turn radius (at least on good-ish terrain) and conserves vehicle speed well during turns.  There were more sophisticated steering systems, but all tanks that had them were only dubiously reliable (e.g. later Brit designs and the big cats).  In field tests with a captured panther, an easy-8 sherman could move over a curving, undulating test track faster than the German tank could despite it having a much more sophisticated steering mechanism.  A notable disadvantage is the tendency to "self-steer" on extremely uneven terrain, which Walter's article notes.

 

-Figure 3 is a clutch-and-brake type, which was historically very common indeed (T-34, KV series, and most German designs).  This is an extremely simple system, but it's not very precise and it loses speed quickly during turns.  Clutch and brake steering does allow one track to be locked while all power is applied to the other track, which allows for an extremely quick, small-radius turn, although not quite as tight as the neutral steer that a double or triple differential system allows.  This was seen by sherman crews as one of the big advantages that German tanks had over theirs; they could spin around in tight positions and GTFO if they needed to quickly.  Some versions of the T42 US experimental medium tank used this type of steering in an attempt to make a transmission that was smaller and simpler than the Allison cross-drive type used in the M46. 

 

-Figure 4 refers to geared steering.  Czech, Japanese, some UK (prior to adoption of the Merritt-Brown steering) and some Soviet tanks in WWII (IS series) used this.  Later the T-54 series used this sort of steering.  The T-54 steering drives are noteworthy because they are combined with the final drives and are extremely compact.  The T95 US experimental medium tank used this type of steering, in an arrangement similar to the T-54, in order to save space and allow the tank to be smaller than the M48 (which used the rather large Allison cross-drive).  

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From the first article:

 

-The steering shown in figure 1 is how a Bren gun carrier steered.  That, and it had pivoting road wheels and flexible track.  This isn't a very good steering system for heavier tanks because it wastes too much energy.

 

-The steering shown in figure 2 is how most American WWII tanks steered; including the sherman, stuart, lee, and pershing.  It is also referred to as the "Cletrac" system, as the Cleveland Tractor Company made a lot of vehicles with this steering system.  Some of the German half-track designs (but none of their tanks) used this steering in conjunction with the pivoting front wheels because GERMAN HALF-TRACK DESIGNS WERE INSANE.  A well-designed Cletrac system was probably one of the better steering systems available for a tank in WWII, since it allows a precise turn radius (at least on good-ish terrain) and conserves vehicle speed well during turns.  There were more sophisticated steering systems, but all tanks that had them were only dubiously reliable (e.g. later Brit designs and the big cats).  In field tests with a captured panther, an easy-8 sherman could move over a curving, undulating test track faster than the German tank could despite it having a much more sophisticated steering mechanism.  A notable disadvantage is the tendency to "self-steer" on extremely uneven terrain, which Walter's article notes.

 

-Figure 3 is a clutch-and-brake type, which was historically very common indeed (T-34, KV series, and most German designs).  This is an extremely simple system, but it's not very precise and it loses speed quickly during turns.  Clutch and brake steering does allow one track to be locked while all power is applied to the other track, which allows for an extremely quick, small-radius turn, although not quite as tight as the neutral steer that a double or triple differential system allows.  This was seen by sherman crews as one of the big advantages that German tanks had over theirs; they could spin around in tight positions and GTFO if they needed to quickly.  Some versions of the T42 US experimental medium tank used this type of steering in an attempt to make a transmission that was smaller and simpler than the Allison cross-drive type used in the M46. 

 

-Figure 4 refers to geared steering.  Czech, Japanese, some UK (prior to adoption of the Merritt-Brown steering) and some Soviet tanks in WWII (IS series) used this.  Later the T-54 series used this sort of steering.  The T-54 steering drives are noteworthy because they are combined with the final drives and are extremely compact.  The T95 US experimental medium tank used this type of steering, in an arrangement similar to the T-54, in order to save space and allow the tank to be smaller than the M48 (which used the rather large Allison cross-drive).  

Agreed on note 2. You were constantly "bumping" WW2 armor.

And yes, the systems on Kraut halfs/semi tracks was batshit. I've dealt with it.

 

I'll mention that a well trained driver can make a braked diff "spin on a dime" . I've done it with the M18, via abusing it's massive automatic transmission and absurd engine. Mash the pedal, honk back the steering brake and hang on.

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On 11/28/2015 at 4:39 PM, Meplat said:

I'll mention that a well trained driver can make a braked diff "spin on a dime" . I've done it with the M18, via abusing it's massive automatic transmission and absurd engine. Mash the pedal, honk back the steering brake and hang on.

 

This sounds like fun.

 

 

I'm sure a bunch of the people here, Meplat in particular, know this already, but here's a quick schematic I did up to show exactly what we're talking about:

tpLeinm.png

 

This is a cross-section of the hull of a panther medium tank with highly skillful MS Paint illustrations to show the different transmission components.

 

Outlined in red is the engine.  The engine is responsible for generating power.  This is either an HL 210 (earliest panthers) or HL 230 (later production) petrol engine.  As you can see, the engine is impressively small relative to its nominal 700 horsepower output.  However, there is generally a trade-off in piston engines between specific power and longevity, and the HL 230 was no exception.  Later in production the engine was downrated to 580 horsepower in order to improve engine life.

 

Outlined in green is the transaxle, which is responsible for taking power from the engine to the transmission.  This piece also has the power take off that powers the turret rotation as well as some universal joints that allow some wiggle room when putting in the engine and transmission components, as German AFV hulls were dimensionally variable.

 

The transaxle is necessary because the transmission and final drives of the panther were in the front of the vehicle, while the engine was in the rear.  This is basically a stupid way to design a tank and there is no justification whatsoever for it.  Contrary to what some sources will claim, front drive sprockets on tracked vehicles do not improve traction.  This mistaken statement is most likely based on a flawed analogy to two-wheel-drive wheeled vehicles, but it does not apply to tracked ones for reasons explained below:

 

NE2vIlS.jpg

 

Outlined in blue is the transmission.  This is a combined transmission and steering drive, except in early panther Ds which omitted the steering drive.  The transmission is responsible for varying the gearing between the engine and the final drives.  The steering drive is responsible for changing the amount of power given to the left and right tracks in order to steer the tank.  Another disadvantage of the panther's frontal transmission is that changing the transmission was a chore:

 

Bundesarchiv_Bild_101I-280-1096-33%2C_Ru

 

The driver and radio operator's stations had to be stripped out, and the transmission changed out through a service cover plate on the top of the hull.

 

Outlined in purple are the final drives and steering brakes.  The final drives are responsible for reducing the RPM and increasing the torque of the power delivered to them before that power is sent to the drive sprockets, which are the toothed round thingies that spin the tracks around.  The steering brakes are an auxiliary form of steering that cut power and slow down the tracks on either side in order to steer.  This is less efficient than the steering drive, but panther drivers were instructed to use this system when possible instead of the steering drive in order to reduce wear on the transmission.

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Freaking outstanding stuff so far, Walter.  I'm working my way through the second article.  There are a lot of interesting bits in there, and many questions raised.

 

-The article states that the moment of inertia of the transmission components is the greatest factor in the initial stages of acceleration of a tank, and that for this reason increasing power to weight ratio only improves intermediate acceleration while improving initial acceleration little.  Why would this be so?  Surely for a given moment of inertia more power would spin it up faster?  Also, does this imply an advantage for pure hydraulic and electrical transmissions?

 

-The article describes the Pz. 61/68 transmission as being continuous power, but not planetary.  HOW THE FUCK DOES THAT WORK!?

 

-The article describes the AMX-30's steering drive as a Cletrac type.  Is this correct?  Every other source I've come across describes it as a purely mechanical triple differential type, later replaced by a double differential with hydrostatic steering drive with the AMX-30B2 upgrade.

 

-The fact that transmissions with hydrodynamic torque converters are not as effective at engine braking downhill is an interesting point that I had not considered before.  I wonder if the Israelis ever wanted the ability to manually select gears and shut out the torque converter when they put Allison CD-850 transmissions in their Sho't Kals?  They did, after all, operate them in mountainous terrain.

-I hadn't noticed before, but the transmission in the Leo 2 is recycled from the MBT-70.

 

-Several sources mention a French firm being contracted to make hydrostatic steering drives for the panther during WWII.  This article mentions hydro steering actually having been fitted to tigers.  First I'd heard of that, and I wonder if they're not confused or mistaken.

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-The fact that transmissions with hydrodynamic torque converters are not as effective at engine braking downhill is an interesting point that I had not considered before.  I wonder if the Israelis ever wanted the ability to manually select gears and shut out the torque converter when they put Allison CD-850 transmissions in their Sho't Kals?  They did, after all, operate them in mountainous terrain.

 

 

 

Does anyone remember that vet's interview where they mentioned that Shermans handled hills better than Pershings in Korea?  They attributed that to the automatic transmission, which I didn't think made much sense.

 

It just occurred to me that the M26 has a torque converter, and the M4 does not.  That would cause problems with engine braking down hills if the M26 was not fitted with auxiliary hydraulic retarders (and I don't think it was so fitted).

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Does anyone remember that vet's interview where they mentioned that Shermans handled hills better than Pershings in Korea?  They attributed that to the automatic transmission, which I didn't think made much sense.

 

It just occurred to me that the M26 has a torque converter, and the M4 does not.  That would cause problems with engine braking down hills if the M26 was not fitted with auxiliary hydraulic retarders (and I don't think it was so fitted).

I do remember that.  It came from here.

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Nice link. Gilbert has a separate mention of Shermans helping Pershings up hills in Marine Corps Tank Battles in Korea:

The Marines had to drive the NKPA out of the valleys and hills, but even some of the gentle slopes presented an unanticipated problem for the American tanks. The M26 was a powerful vehicle, but as the tank crews soon discovered, if it stopped on a steep gradient the transmission would slip, and it was difficult or impossible to get it moving again. Help was needed and Eugene Viveiros, who, with one of the Headquarters Platoon blade tanks, attached himself to 3rd Platoon, was ready to supply it. He was called upon "to pat them on the butt end with the blade of the 'dozer tank to shove 'em up and get 'em going again. Once they got traction, then they were all right," Viveiros explained.

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Here are some listings of older engines.

 

Table+36.+Main+characteristics+series+en

Thanks, I have been wondering how much power and how big a electric engine would need to compete with a internal combustion engine. 

 

http://www.engineeringtoolbox.com/electrical-motors-hp-torque-rpm-d_1503.html

One thing that confuses me about this table is the torque output of a 600hp engine at 1000RPM, which is a crazy 4725Nm.

So, is the gearbox able to convert the RPM in the engine to more torque to reach such numbers? 

So for example the MB-838 with its 1500RPM and 2744Nm could become 1000RPM and 4116Nm? (2744*1,5=4116)

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I am for the most part thinking about the electric engine, in which you can use this formula to calculate the torque:

Torque in SI units can be estimated as

 

T = PW 9.549 / n         

where

T = torque (Nm)

PW =  power (watts)

n = revolution per minute (rpm)

 

When it comes to a conventional engine, I am fully aware that the peak torque and horsepower does not follow the same curve. I was thinking about how a gear box works, you know, let's say you have a fast spinning low torque axle, and you want it to spin slower. Then you can simply do this right?:

ftgears2.jpg

 

This would cause the larger cog to spin slower, and also have more torque.  So since the MB 838s optimum RPM for maximum torque is 1500RPM, but the electric engines torque is 1000RPM, we could use a gear box to convert some of the RPM to torque, so that both run at 1000RPM at the axle. I think they are called gear ratios. 

 

​(Gah, just realized that we have a transmissions and final drives thread, probably should have posted the question there >_<)

 

 

Yes.  Power is torque x rotation speed.  I'm sure that somewhere out there is a derivation of this.  A gearbox trades off torque for more RPM or RPM for more torque, albeit at some efficiency loss.

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Help me out with terminology, tank gurus. The strv m/42 (and strv 74) is steered with two levers. Pulling one of the levers, says the manual, gears that track down to 2/3, and the tank turns in that direction, giving a fixed turn radius of about 9 meters. To avoid burning up the brake discs in the steering gears, you steer by yanking on the steering levers firmly - don't try to "slip on the clutch" so to speak. However, if you want to make a really tight turn, you can also press a button on top of the steering levers to instead completely brake the inner track and turn around a point about half a meter inside the inner track. This is only allowed on low gears (first, second and reverse). If you want to start the tank uphill or something and the engines stall on you when trying to get into gear, you can try pulling both the steering levers back which gears down by 2/3 as previously mentioned and gives you a bit more pull.

Is this what you call "geared steering" in English, with an additional clutch-brake system for the button on the steering levers?

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Help me out with terminology, tank gurus. The strv m/42 (and strv 74) is steered with two levers. Pulling one of the levers, says the manual, gears that track down to 2/3, and the tank turns in that direction, giving a fixed turn radius of about 9 meters. To avoid burning up the brake discs in the steering gears, you steer by yanking on the steering levers firmly - don't try to "slip on the clutch" so to speak. However, if you want to make a really tight turn, you can also press a button on top of the steering levers to instead completely brake the inner track and turn around a point about half a meter inside the inner track. This is only allowed on low gears (first, second and reverse). If you want to start the tank uphill or something and the engines stall on you when trying to get into gear, you can try pulling both the steering levers back which gears down by 2/3 as previously mentioned and gives you a bit more pull.

Is this what you call "geared steering" in English, with an additional clutch-brake system for the button on the steering levers?

 

Yes, single-radius geared steering with auxiliary clutch and brake steering.  Very comparable to how a T-54 steers.

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      We'll take some whacks at everyone's favorite whipping boy; the Panther.
       
      A US report on a captured Panther Ausf G gives its top speed on roads as an absolutely blistering 60 KPH on roads.  The Soviets could only get their captured Ausf D to do 50 KPH, but compared to a Sherman, which is generally only credited with 40 KPH on roads, that's alarmingly fast.
       
      So, would this mean that the Panther enjoyed a mobility advantage over the Sherman?  Would this mean that it was better able to make quick advances and daring flanking maneuvers during a battle?
       
      No.
       
      In field tests the British found the panther to have lower off-road speed than a Churchill VII (the panther had a slightly busted transmission though).  In the same American report that credits the Panther Ausf G with a 60 KPH top speed on roads, it was found that off road the panther was almost exactly as fast as an M4A376W, with individual Shermans slightly outpacing the big cat or lagging behind it slightly.  Another US report from January 1945 states that over courses with many turns and curves, the Sherman would pull out ahead because the Sherman lost less speed negotiating corners.  Clearly, the Panther's advantage in straight line speed did not translate into better mobility in any combat scenario that did not involve drag racing.
       
      So what was going on with the Panther?  How could it leave everything but light tanks in the dust on a straight highway, but be outpaced by the ponderous Churchill heavy tank in actual field tests?
       

      Panther Ausf A tanks captured by the Soviets
       
      A British report from 1946 on the Panther's transmission explains what's going on.  The Panther's transmission had seven forward gears, but off-road it really couldn't make it out of fifth.  In other words, the Panther had an extremely high gear ratio limit that allowed it exceptional speed on roads.  However, the Panther's mediocre power to weight ratio (nominally 13 hp/ton for the RPM limited HL 230) meant that once the tank was off road and fighting mud, it only had a mediocre power limit.  Indeed, it is a testament to the efficiency of the Panther's running gear that it could keep up with Shermans at all, since the Panther's power to weight ratio was about 20% lower than that particular variant of Sherman.
       
      There were other factors limiting the Panther's speed in practical circumstances.  The geared steering system used in the Panther had different steering radii based on what gear the Panther was in.  The higher the gear, the wider the turn.  In theory this was excellent, but in practice the designers chose too wide a turn radius for each gear, which meant that for any but the gentlest turns the Panther's drive would need to slow down and downshift in order to complete the turn, thus sacrificing any speed advantage his tank enjoyed.
       
      So why would a tank be designed in such a strange fashion?  The British thought that the Panther was originally designed to be much lighter, and that the transmission had never been re-designed in order to compensate.  Given the weight gain that the Panther experienced early in development, this explanation seems like it may be partially true.  However, when interrogated, Ernst Kniepkamp, a senior engineer in Germany's wartime tank development bureaucracy, stated that the additional gears were there simply to give the Panther a high speed on roads, because it looked good to senior generals.
       
      So, this is the danger in evaluating tanks based on extremely simplistic performance metrics that look good on paper.  They may be simple to digest and simple to calculate, but in the messy real world, they may mean simply nothing.
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