Michigan Company

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CLARK POWER SHIFT TRANSMISSIONTheory of Operation

Why do you need a transmission at all? Well,it's because an engine all by itself just won'tcut the mustard. The work we do with construc­tion machines requires high speeds and lowspeeds, but enough torque at any speed.

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run it slower than that range and it just doesn'thave the power. It might be possible to buildan engine with plenty of torque over a wideR.P.M. range, but it would have to be so big

3M 8411 CGAC

The problem is that an engine really only putsout within a certain R.P.M. range .. '.

"RPM

that you'd need King Kong to operate it.

So we use a transmission to keep that engineat the proper R.P.M. while we're doing work thatwould otherwise be too slow or too fast for theengine. The transmission gives us the flexibilityto do a variety of speeds.

So, what is a transmission?

And this transmission gives you three big plusses.

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It's a set of gears and shafts which transmitenergy from the engine to the driving wheelsof a machine.

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First, the machine doesn't always have to berolling while the engine is running. You putthat transmission in neutral and the engine canturn over even though the machine is standingstill.

And, when you need speed, you have a low gearratio or high gear.

Second, when you need a lot of power and a lotof torque like for going up a steep grade orpushing something heavy you have a high gearratio in the transmission. This high gear ratiois commonly called low gear.

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There are all kinds of transmissions. There arethe sliding gear and syncromesh types which usesliding gears and a mechanical friction clutch.

Clark Power Shift Transmissions are like an auto­matic transmission, in that the gears are con­stantly meshed, and the power comes from atorque converter. But instead of clutch bands,a power shift transmission has clutch discs whichengage the gears and these hydraulically op­erated clutches are not activated by some sortof mechanical governor, but are activated by theoperator himself.

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And there is the automatic transmission, whichuses hydraulics to actuate clutch bands. In thistransmission, the gears don't slide but are al­ways meshed. A governor control senses thespeed and load and directs the hydraulic oil towhichever clutch bands give the best gear ratiofor the work being done.

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To smooth out the process, this type of trans­mission gets its power from a fluid coupling.

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As you can see, if you understand these hydraulicdisc clutches, you will have a pretty good under­standing of the power shift transmission ingeneral.

As you can see, a disc like this could slide backand forth in the grooves but could not rotateindependently of the shaft.

Let's build one of these clutches from the groundup and you'll see some clever engineering.Imagine a shaft with grooves cut in its surfaceand a disc with tongues that fit the grooves.

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Now imagine a cylinder with grooves cut in itsinside surface and a disc with tongues to slidein these grooves.

"DISC

CYLINJ:E1<,

If you were to take the grooved shaft and placeit within the grooved cylinder and alternate discs,so that one disc is tongued to the cylinder whilethe adjacent one is tongued to the shaft, youwould have the beginnings of a clutch.

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Once again, the disc can slide back and forthbut because of the tongue in groove arrangementthe disc cannot rotate independently of thecylinder.

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•S-l-lAFTDISCS

CYL.fNteC'DISCS

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All of the discs in this mechanism can slide backand forth, but some discs turn only with thecylinder while others turn only with the shaft.

Here are two pistons in an oil filled cylinder. Ifyou press down on piston A the oil transmitsthe pressure to all interior surfaces, but the onlysurface that gives in to that pressure is the bot­tom of the other piston, piston B.

If a piston is installed at one end, and a path isprovided for high pressure oil, then the mechan­ism is completed. Imagine that the cylinder isturning and the shaft is not. If the oil forcesthe piston against the discs, the discs presstogether forming a sandwich ... a friction sand­wich. The cylinders discs, pressing the discs onthe shaft cause the whole mechanism to turn asone unit. And that's how a hydraulic clutchworks.Clark has refined this basic mechanism and youshould understand some of these refinements.But before you can understand refinements youneed to know a few basic principles of hydraulics.

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If both pistons are the same size, when you pressdown on piston A then piston B will rise out ofthe cylinder just as fast as A is depressed, andwith just as much force.

If the bottom surface of piston B is greater thanpiston A, when A is depressed, B will rise out ofits cylinder more slowly than A is depressed ...but with more force. Here you're getting forcein exchange for speed. You can see that theamount of piston surface determines how fastthe piston moves and with how much force.

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If piston B is smaller than piston A, when youpress down on A, smaller piston B will rise outof the cylinder faster than A is depressed, butwith less force. You're trading speed for force.

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Now you're ready to understand Clark's steppiston. The piston within has two surfaces.When oil is directed to the piston, it initiallyacts only on the step piston's small surface. Thepiston moves rapidly toward the discs, butwithout maximum force.

,ADDITIONALSURFACE.

First, you have to release the oil pressure frombehind the piston. This permits the piston returnsprings, which have been compressed by themoving piston, to push the piston away from thediscs. But releasing the oil pressure doesn't dothe whole job. Some of the oil can escape backthrough the inlet passage.

As the squeezed discs cause resistance to pistonmovement, the oil is forced around the edges ofthe small piston surface and begins to act on theadditional surface, providing very great force.You will find that some clutches use a step uppiston and some use a step down piston. Eitherway, the same principle is involved, the greaterthe surface the greater the force.

So the Step Piston design patented by Clarkfunctions with clutch disc resistance to providequick, smooth, and powerful application of force.So far we've only talked about applying theclutch. Let's look at the way the clutch isreleased.

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But because the drum is spinning rapidly, theoil flows with centrifugal force toward the out­side of the drum making escape through thecentral inlet passage difficult. But if centrifugalforce causes the problem, then centrifugal forcecan solve the problem.

The ball rolls up the ramp because of centrifugalforce, revealing an escape passage for the oil.This ball permits quick release of the clutch.

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There is a check ball, fitted in a ramp at theouter edge of the drum. •

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When the clutch is reapplied the high pressureof the oil forces the check ball into the neckof the escape passage preventing a loss inpressure.

So now you know about a couple of refinementsClark has engineered into the hydraulic clutchof a Powershift Transmission: the Step Piston,and the Ball Check Valve. But up till now you'vebeen looking at a very simplified diagram ofthe clutch .

the small and large surfaces of the step piston

Here is a diagram to give you a better idea ofwhat the real thing looks like.

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the discs -

the inlet passage-

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•the piston return springs-

•--- -- - -- ---- -,--,.-.-.--..------------------

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and the check ball escape passage.

When pressure is released oil escapes back outthe inlet passage, but also through the centri­fugally operated check ball valve.

Here is a review of how the oil activates theclutch. High pressure oil is sent through the oilinlet passage forcing the piston into the discsand compressing the piston return springs.

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A power shift transmission is nothing more thana series of these clutches which lock gears andshafts together to select various gear ratios orneutral. This Powershift Transmission has oneclutch for each of the four speeds, plus oneclutch for forward mode, and another for reverse.So a four speed transmission has a total of sixhydraulic clutches. And anytime the vehicle ismoving, two of the clutches will be engaged:either forward or reverse clutch and one of thefour speed clutches.

kind of like a traffic cop sending oil down onepath and stopping it from traveling down anotherpath.

First, let's talk about how the valve regulatespressure.

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~"i;iiiiei+iijiljjijWijji iiiiiiiiiii ~-----;;;;;;;-;-------------

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These clutches are activated through a devicecalled the transmission control valve, which you •will find mounted on top of the transmissioncase. This control valve does two things - itcontrols the amount of 0 i I pres sure to theclutches, and, it permits or prevents oil flow tothe various clutches,

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Oil is pumped to the valveby a geartype pump,sometimes called the converter charge pump.But the clutches operate best only within alimited pressurerange.

The spring which is holding tension against thespool is what regulates clutch pressure in thetransmission.

The pressure regulator valve consists of a har­dened valve spool operating in a closely fittedbore. Oil entering the transmission from thecharge pump must pass through the regulatorvalve. After passing through the valve, it willapply a speedclutch and stop. When oil flow isstopped at the clutch, pressurebuilds and oilflows through a passagebehind the spool forcingthe spool to move against the spring. As thespool shifts, it opens a port which allows theexcessoil to charge the torque converter. Thisall happens within a fraction of a second.

FROM PUMP

TO CLUTC+-FESTO CONVERTER

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Very similar devices are used to activate thevarious clutches.

In this position the reverse clutch is activated.

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After oil passes the pressure regulator spool, itflows to the forward/reverse spool. When thespool is in this position, the forward clutch isactivated.

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This positio gives you neutral.

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Right next door is the spool which gives youfirst, second, third, or fourth.

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Second ... ,

or Fourth ... And because the forward reversespool is independent of the speed selectionspool, you can have any of these speeds in re­verse as well as in forward.

So you can see that there are basically threespools involved in the transmission control valve:

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•Third ... ,

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First the spool that regulates pressure,

and the one that gives you speed selection.

You'll find more detailed powerflow diagramsfor each powershift transmission in its shopmanual.

the one that gives you forward or reverse,

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Well ... that's about it. A power shift transmis­sion is nothing more than a series of clutcheswhich lock gears and shafts together to selectvarious gear ratios and forward or reverse mode,

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and this selection is accomplished through thetransmission control valve which sends oil towhichever clutches are selected by the operator.

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