8 GEARS November / December 2005

During the recent ATRA's Power-train Expo in Las Vegas, I provided a seminar on Basic Hydraulics. I spent months preparing for the class, and when all was said and done, I think I was the one who had learned most about the subject. Though it could be that the greatest revelation was how difficult it is to teach 250 technicians about hydraulics in only 90 minutes!

I’ve since decided to take more time, and present the topic in a series of articles here in GEARS magazine. I’ll start with the most fundamental issues related to hydraulics, and con-tinue through to the most advanced. Some of the articles may seem too basic for some readers, while others may seem very advanced. For the majority, though, I think the series will be a good, informative review.

So … welcome to Basic Hydraulics. In this edition we’ll start right where itall begins; at the pump.

The first thing to keep in mind is that pumps don’t create pressure. They create volume. Just like when you breathe out through your mouth, the air doesn’t create pressure; it cre-ates volume or flow. If you blow that same breath into a balloon, however, pressure is created. A pump merely transfers transmission fluid from the pan, through the pump, and forces the fluid into circuits throughout the trans-mission. Once the oil is pumped into these circuits and gets trapped, it then becomes pressure.

There are many types of pump configurations used in automatic trans-missions, but they can be divided into four basic types: internal gear type, rotor type, gear type and vane/rotor.

Internal Gear TypeThese are the most common pumps

used throughout our industry. For this example we’ll look at a pump from a THM 400 (figure 1). For this discus-sion, the major parts are the internal gear, the outer gear, the pump bore and the crescent. When the internal gear is turned by the engine, it rotates the outer gear. Because the gears are offset and separated by the crescent, oil is pulled into one side (the intake side) and car-ried by the area between the teeth of both gears and transferred to the other side (the output side). When the teeth mesh, they squeeze the oil to the out-put side of the pump. The bigger the diameter and/or the thicker the gears, the more oil volume the pump will pro-duce. This is true with all pumps.

Now I realize this seems very basic and simple. It is. However, this is why a lot of problems are often overlooked. How many times have you had a come-back in which the problem was solved after you replaced the pump? Since the primary job of the pump is to transfer oil, it will do so the easiest way it can. It’s much easier to transfer oil back into the intake side of the pump, because the pressure there is less than on the output side. For example, a worn crescent will often allow a lot of oil to leak between the teeth of the internal gear and the crescent, back into the intake side of the pump. Another area for pump volume loss is with excessive gear clearances. Always look for wear, and check all pump clearances to factory specifica-tions.

BASIC HYDRAULICS

by Larry FrashYou You UpUpPumpPump

Figure 1

InternalGear

OutputSide

Crescent

ExternalGear

IntakeSide

Oil is pulled into the intake side and carried by the area between the teeth and transferred to the output side.

Internal Gear Type

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10 GEARS November / December 2005

Rotor TypeA good example of a rotor type

pump is a Torque Flite (figure 2). This pump operates the same way as the THM 400 type with one major differ-ence. There is no crescent to divide the intake from the output side of the pump. The Lobes (rounded teeth) of the internal and external gears act as the crescent. Take a good look at these lobes next time you work on a pump with this design. You’ll almost always find some, if not a lot of wear, on them. Yet these gears are used over and over again. We’ve all done it! Lobe wear has the same effect as a worn crescent in an internal gear type pump.

Gear TypeThe most common example of

this type of pump is one from a Honda Transaxle (figure 3). As the gears rotate, oil is pulled into the intake side of the pump, carried by the area between the teeth, and transferred to the output side. There is no crescent, however. The outer walls (bore for the gears) act as the crescent. The clearance between the teeth and the bore must be close enough to seal the pump intake from the pump output. Due to the small size of these pumps, even a brand new pump is not very efficient. When these pumps have even small amounts of wear, they can cause problems.

With all gear type pumps, the volume of oil they pump only changes with pump RPM. Because pumps are designed to produce adequate volume at idle speeds, gear type pumps will produce too much volume at higher RPM. So at these higher RPMs, the pressure regulator valve must be able to exhaust much of the excess volume. If the excess oil isn’t exhausted prop-erly, a high line pressure condition will occur.

Vane/Rotor TypeThere are several differences

between a gear type pump and a vane/rotor type (figure 4). The obvious dif-ference is that there are no gears. The rotor rotates several vanes with rings that keep the vanes pushed against the slide; not unlike a paddle-wheel. Because the slide is offset of the rotor centerline, oil is pulled in from the

Pump You Up

Figure 2

Figure 3

Figure 4

OutputSide

Oil is pulled into the intake side and carried by the area between the lobes and transferred to the output side.

Rotor TypeThe area where the lobes almost contact acts as the crescent

IntakeSide

Oil is pulled into the intake side and carried by the area between the teeth and the outer walls and transferred to the output side.

The outer wall acts as the

crescent

Gear TypeIntakeSide

OutputSide

The outer wall acts as the

crescent

Vane/Rotor Type

Vanes Slide

RingRotor

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12 GEARS November / December 2005

intake side and carried in the area between the vanes, then transferred to the output side of the pump. When the slide is in the max volume position, the area between the vanes that carries the oil is very large compared to the smaller area where the rotor almost contacts the slide (figure 5). This produces a very large amount of volume. As the pump RPM increases, the pressure regula-tor will act by sending pressure to the decrease side of the slide. This will move the slide to a lower volume posi-tion (figure 6). In this position much of the oil that is carried between the vanes is simply delivered back to the intake side of the pump. This pump design allows variable pump volume. Because of this, the pressure regulator valve is not designed to exhaust pump volume. If the slide sticks in the maximum vol-ume position, mainline pressure can easily exceed the limit of the aluminum pump body.

These pumps have wear problems similar to gear pumps, but they also have their own unique problems. The vanes commonly wear in the area in which they contact the rings. When this happens, the vanes won’t prop-erly contact the slide, which creates an internal leak and a volume loss. Rotor clearance is also very critical. The rotor has to seal three separate circuits: pump intake, pump output and converter charge (in the center of the rotor). There are many more areas that wear in these pumps, just keep in mind that any place that is worn will cause a low volume condition.

A problem that is unique to vane and rotor type pumps occurs when the slide sticks. If the slide sticks in the low volume position, you’ll have low line pressure at idle. If the slide sticks in the high volume position, you’ll have uncontrollable line pressure at high pump RPMs. A good way to check for a

sticking slide during the rebuild is to do what I call the shake test. Simply install the slide and pivot pin into the body and torque the two pump halves together. Shake the pump side to side and you should hear a clink- clink sound if the slide is free.

As you can see, even though pumps are very simple in design, a small

flaw can make them useless. Always check for wear and pump clearances using factory specifications during each rebuild. We’ll cover more in the next edition of Basic Hydraulics.

Pump You Up

Oil is pulled into the intake side and carried by the area between the vanes and the slide and transferred to the output side.

Vane/Rotor TypeMax Volume Position

OutputSide

If the slide sticks in the maximum volume position, mainline pressure can easily

exceed the limit of the aluminum pump body.

Figure 5

IntakeSide

Vane/Rotor TypeMinimum Volume Position

OutputSide

When the slide is the minimum volume position, much of the oil is simply delivered back to the intake side.

Figure 6

IntakeSide

ConverterCharge

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