Auto Shop Tech Rear Axle http://www.autoshoptech.co.cc Tue, 08 Nov 2011 20:04:20 +0000 http://wordpress.org/?v=2.9.2 en hourly 1 Auto Shop Tech Rear Axle http://www.autoshoptech.co.cc/2010/05/26/autoshoptech/ http://www.autoshoptech.co.cc/2010/05/26/autoshoptech/#comments Thu, 27 May 2010 02:44:12 +0000 admin http://autoshoptech.co.cc/?p=1 All about rearend gears-how they work, setting them up, and their wear patterns

For a moment, think of engine power as being capable of flowing like water. It travels from engine to driveline, down the driveshaft and out into the rear wheels. As it passes from the end of the driveshaft out into each rear axle, it moves through a combination of gears called a ring and pinion. This set of gears changes the direction of power flow from the axis of the vehicle (driveshaft) into the axes of the rear wheels. And it usually does this along with a change in turning ratio, driveshaft to wheels.

For example, a 3.7:1 rear gear ratio means that for each revolution of the rear axles, the pinion (or drive) gear will rotate 3.7 times. And the more pinion gear revolutions per one complete turn of the ring gear, the less inertial resistance the vehicle offers to acceleration. In other words, a 4.56:1 rear gear ratio allows quicker acceleration than a 3.5:1. But for purposes of outright fuel economy, you’d probably want a ratio in the 2.9:1 range, since net engine rpm and fuel used are closely related.

The determination of rear gear ratios is a relatively simple task. Pinion gears have fewer teeth than ring gears. And, as stated earlier, the rotational relationship between the two is based on how many times the pinion gear revolves for one complete turn of the ring gear. If, for example, a given pinion gear has 12 teeth and its ring gear has 42, the ratio between the two is 42/12 = 3.5:1. The more revolutions the pinion makes for one turn of the ring gear, the less resistance offered to increases in engine rpm; so a vehicle equipped with a 4.56:1 ring and pinion gear set will accelerate more quickly than a similar car fitted with a ratio of 2.9:1. Not all that complicated, but an ingredient in what this gear business is all about.
Of the several designs associated with ring and pinion gears, the three most common to automotive applications are (1) straight bevel, (2) spiral bevel and (3) hypoid.

Straight bevel and spiral bevel gears (Figure A and Note 1) are of a design in which the centerlines of ring and pinion gears intersect. Also, the angle (called the “shaft angle”) between pinion axis and ring gear axis is normally 90°. The angle of the teeth (called the “pitch surface”) for each gear lies in a cone (Figure F). As the teeth in a straight bevel mesh, there is very little (if any) sliding motion in tooth contact. This tends to make this gear design more noisy with high initial load shock as compared to either the spiral bevel or hypoid method.
Spiral bevel gears have curved teeth but with a pinion and ring gear relationship like straight bevel gears (axes intersecting). Also, as spiral bevel teeth come into contact with each other, contact begins at one end of a tooth and moves to the other as the gears turn. This is a particularly nice feature for ring and pinion gears required to operate at high speeds, since both noise reduction and uniformity of tooth loading are improved.

A. Bevel ring and pinion gears are characterized by a ring and pinion whose axes intersect (the centerline of one intersects with the centerline of the other). In automotive applications, the angle between these centerlines (the shaft angle) is normally 90°. If the ring and pinion teeth were cut obliquely, as in note 1, the gear set would be a “spiral bevel” design. Such a design provides tooth contact that begins at one end of each tooth and progresses to the other end. This results in smooth, quiet application of power from pinion to ring gear. B. Popularity of spiral bevel gears (for automotive applications) decreased in the 1930s, giving way to the hypoid design. Although similar in appearance to spiral bevel gears, hypoids are characterized by such pinion gear location that the gear axes do not intersect. This allows lowering of a vehicle’s driveshaft and reduced overall car height. And hypoid gears are stronger than spiral bevel gears. For you automotive engineering types, this is because the spiral angle of the pinion is larger than the spiral angle of the ring gear.

The third basic ring and pinion design (hypoid gears) has pretty much become the industry standard. Although much like spiral bevel gears, hypoid gears employ pinion gear location that is below the centerline of the ring gear (Figure B). The axes of both gears, therefore, do not intersect. To compensate for this offset and to maintain the proper amount of tooth contact, the spiral shape of the gear is changed to more of a hyperbolic curve. Now before you slash your wrists (or flip back up to the Post Entry section), let’s define a hyperbola as the shape of a curve that you’d get by slicing a cone from top to bottom (but not through its axis) as shown in Figure G. As compared to either the straight bevel or spiral bevel gear design, hypoid gears are cut so that the angle of the teeth lies along a hyperbolic curve. Now if all this seems to be getting a little complicated, just remember that we never said that it wouldn’t get that way. Besides, we didn’t name the gears; somebody else did.
At any rate, hypoid gears are characterized by centerlines that don’t

intersect (ring and pinion gears) and tooth design that follows hyperbolic curves. This design is both quiet and strong. And it also lowers the position of the driveshaft, allowing lower overall vehicle height. The teeth of hypoid gears contact each other in a sliding action, rather than the harsh, full-tooth loading that is characteristic of other designs.
In terms of ring and pinion gear installation, there are two basic terms to keep in mind: (1) pinion depth and (2) backlash. Pinion depth is the measured distance from the back of the pinion gear to the centerline of the ring gear. This you can see in Figure D. Some gear manufacturers call this the “checking distance,” but it amounts to the same thing as pinion depth.
Variations in the amount of pinion depth can be made by the addition (or removal) of washer like shims usually placed between the forward side of the pinion gear and the back of the pinion bearing (Figure D). Some amount of pinion bearing pre-load is normally required, but this really isn’t affected by the amount of pinion depth shims. Just shim to the manufacturer-suggested depth and pre-load the bearings (using a “crush sleeve” or alternative factory method).

C. Backlash is a term applied to the amount of air gap between ring gear and pinion gear. A dial indicator should be positioned so that tooth movement is roughly perpendicular to the direction of indicator travel. Following proper setting of the pinion depth (as shown in Figure D), backlash can be adjusted by movement of the ring gear into or out of the pinion (side bearing pre-load). D. Pinion depth establishes the tooth contact pattern between ring and pinion gears. High-performance gear sets are usually marked with a setting for both pinion depth and backlash’. Therefore, such gears should be installed as sets, not mixed with comparable ring and pinion gears. To adjust pinion depth, shims should be placed between the pinion face and bearing or in front of the bearing (depending upon design of rear gear housing).

After installation of the pinion gear, backlash can be checked with the ring gear in place. And to make certain that misalignment of ring and pinion gears doesn’t exist, backlash checks should be made in at least three equally spaced locations around the perimeter of the ring gear. Positioning of a dial indicator (as shown in Figure C) will indicate the amount of backlash. Most gear manufacturers suggest no more than .003-inch variation from factory specifications. For example, if a particular gear set is to be installed with a backlash of .006-.010-inch, there should be no less than .003- or more than .013-inch. Side shim packs or adjusting rings provide backlash variation.

E. Most stock rear gears have a power-and-coast (loaded and unloaded) tooth contact pattern in the center of the ring and pinion gears. Some high-performance gear sets require a toe-contact pattern. Actual tooth contact can be checked using red lead paste by coating a portion of the ring gear and rotating the gear set by hand. F. This shows the relationship between ring and pinion gears in a typical straight or spiral bevel design. Note that centerline of each gear intersects at a common point and that both gears share a common apex for pitch angle cones.

H. This is a typical differential and pinion gear arrangement. By the addition of a clutch pack between the axle side gears (c), a positive lock-up of both rear wheels is achieved for straight-ahead movement of the vehicle. Note, in either type of unit, the pinion housing, or carrier or case, as it is sometimes called, (a) is bolted to the ring gear, causing the entire assembly to rotate in the plane of the ring gear. It is only when one axle turns at a different rate of speed that the differential pinion gears (b) rotate about their shaft (d). I. During a turn, the outside wheel will turn faster than the inside wheel, because it has farther to go in the same amount of time as the inside wheel. This “difference” in wheel speed is compensated for by the differential gears (b) & (c).

Once you’ve satisfied the manufacturer’s recommended installation clearances, it’s wise to check gear tooth contact patterns to make certain the housing you’re using (or some other influential factor) hasn’t upset proper tooth mesh. To do this, mix a paste from powdered red lead (obtainable at your local paint store) and a little clean engine oil and paint the material onto both ring and pinion gear teeth. Rotation of the pinion gear (in the direction of normal rotation) wil produce a tooth contact pattern comparable to what the gear will see in use. Admittedly, you’ll not load the gears as severely as in actual operation, but a contact pattern showing pressure toward the toe end of the teeth will satisfy most model vehicles (with the exception of AMC and Dana-type gears which have contact patterns located nearer the center of the teeth).
In cases where you are not able to establish the correct contact patterns, some amount of adjustment of either pinion depth or ring gear backlash may be required. Just keep in mind that you should not exceed the usually recommended .003-inch variation in factory specifications for backlash and .002-inch for pinion depth. For gear sets not supplied with recommended pinion depth and backlash settings, use the red lead contact pattern method and you’ll likely have no gear performance problems.
In addition to the theory of ring and pinion gears, there is also the relationship this type of gear has with the business of moving your car. And it has to do with how driveshaft torque is converted into motion for the two rear wheels. What it boils down to is trying to make each rear wheel turn at the same rate of speed. Most often, this is a problem associated with going around corners, when the inside wheel tends to turn at a slower rate than the outside wheel.
This “differential” rate of turning is accomplished by an interaction of pinion gears located on a common shaft and side gears or “axle” gears into mesh with these two pinion gears.
A fundamental difference between conventional and non-slip differentials is that the “limiting slip” design utilizes a clutch pack (series of discs and friction surfaces) whereby the slippage of either rear wheel causes the clutch pack to transmit torque to the wheel turning at the slower rate. It is the ability of non-slip differentials to equalize rear wheel torque (between the two driving wheels) that makes it possible for each wheel to provide the same, or about the same, amount of driving force.

Traditionally, a common failing of standard ring and pinion gear sets has been the inability to divide rear wheel torque equally to each axle shaft. When one wheel spins, the other doesn’t—thus the need for non-or limited-slip differentials.
In pure theory, such differential gears are actually a form of planetary gears. According to Figure H, you can see that the pinion gear drives the ring gear. This ring gear is bolted to a housing (a) which also carries the pinion gears (b) and axle side gears (c). For straight-ahead motion of the car, the axle side gears and pinion gears rotate as a unit with the housing. But when the car makes a turn or one wheel tends to rotate faster than the other (mud, ice, gravel, or other such slippery circumstance), the axle side gears no longer tend to rotate at the same speed. The use of a set of pinion gears connected on a common shaft and meshed with the axle side gears allows this differential rate of turning. It also allows for such extremes as rear wheels turning in opposite directions, a condition that could result from a vehicle spin without power.
It might be interesting to note that in a conventional differential gear assembly, holding one rear wheel fixed will allow the other to turn at a speed twice the speed of the planetary gear housing (carrier). That’s a little bit of information you won’t be asked to recall on the test, but you might think about it.

In high-performance applications, a common deviation from conventional or limited-slip differentials is the removal of the differentiating and replacement with a so-called “spool” which effectively locks both rear axles together. Each, therefore, tends to turn at the same rate. But if, for some reason, either rear wheel tries to turn faster (or slower) than the other, there’s a good chance the vehicle will make a sudden change in direction (left or right). And perhaps the most common situation here is when a rear axle breaks during hard acceleration.
This month, by way of quick review, we’ve looked into the more common ways engine torque is converted into a car’s driving wheels. Ring and pinion gear sets allow this change in power flow direction. And through the use of a small planetary gear set and housing called a differential (often in conjunction with limited-slip capability), an equalization of rear wheel torque is approached or compensation made for different wheel speeds (one relative to the other). It was mentioned earlier that power flows through the drivetrain like water. Hopefully, it was a concept intended to aid in the understanding of rear gear operation and not one that was all wet.

REVIEW QUESTIONS: True or False
1. Hypoid gears are no longer the popular choice of design for automotive applications.
2. Typically, pinion gears have more teeth than ring gears.
3. Spiral bevel gears allow for lower overall vehicle height, since the centerlines of ring and pinion gears do not intersect.
4. Pinion depth is critical and is measured from the back of the pinion gear to the rear of the differential housing.
5. Of the various types of ring and pinion gear designs, the hypoid gear set is the strongest.
6. In order to reduce gear noise, all of the tooth contact surface should come into contact with the meshing gear at the same time.
7. Backlash measurement can easily be made by rotating the pinion gear by hand and noting the number of degrees of rotation.
8. Gear ratio is a mathematical comparison of the number of teeth on the ring gear vs. the number of teeth on the pinion gear.
9. A 3.7:1 ring and pinion set means that for one revolution of the pinion gear, the ring gear will turn 3.7 revolutions.
10. A car equipped with a 2.9:1 rear gear ratio will accelerate more easily than a comparable vehicle fitted with a ratio of 3.7:1.
11. In a typical non-slip differential, there is no need for pinion gears, since torque is transmitted directly through the ring gear.
12. The clutch pack (in a non-slip differential gear set) causes pressure to be applied to axle side gears, thus dividing torque between the two rear wheels.
13. Hypoid gear teeth are less prone to cavities than spiral bevel gear teeth.

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