Glossary

Alloy Steel

Steel containing significant amounts of elements (besides carbon and certain amounts of trace elements) added to change the mechanical or physical properties of the steel. Specifically, steel is considered an alloy when it contains more than the percentages listed of one or more of the following elements: 1.25 percent manganese, 0.60 percent silicon, 0.60 percent copper, more than 3.99 percent cobalt, columbium, molybdenum, nickel titanium, tungsten, vanadium, zirconium or any other alloying element.

Annealing

If the steel becomes too hard, it can be softened, or annealed, to a desirable hardness. This is done by heating to a predetermined point (according to the material) followed by a slow cooling. This process is also used to relieve the internal stresses in a part.

Axle Hop

See Axle Wrap

Axle Shaft

The forged steel bar that connects the differential to the wheels. It transmits torque from the differential to the wheel and may also support the weight of the vehicle.

Axle Wrap

Torque and traction combine to twist the axle (pushing the nose of the differential up and going forward and down going backward) by twisting the leaf spring into an “S” shape. The energy is stored in the spring until the tire slips, and at that point the spring snaps back violently. Sometimes this results in a hopping sensation, hence the other common term, “axle hop”. It can be very hard on driveshafts and u-joints.

Bearing

Used to minimize friction in moving parts. They may also support weight or thrust loads.

Billet

A part machined from a forged piece of metal rather than a casting.

Brittleness

The tendency of a metal to shatter or break rather than bend, twist, or otherwise change shape under a load.

Cardan Joint

The original name for the common universal joint, named for the 16th century Italian mathematician, Jerome Cardan, who established the basic principle of operation.

Carrier

The carrier, or carrier case, is the unit which the ring gear bolts to and axles slide into. The carrier case transfers power from the ring gear to the axles.

Case Hardening

This is a process where the outer shell or case of the metal is hardened only to a precise and predetermined depth.

Caster

The forward or backwards tilt of the steering axis. Positive caster tilts the top pivot from the vertical toward the rear of the vehicle and negative does the opposite.

C-Clip

A C-clip is a semi-circular retaining clip used to hold semi-float axles in the housing. With the cross spin shaft removed, the axle can be pushed into the housing far enough for the C-clip to be installed into a groove on the end of the axle splines. The axle is then pulled back outward until the C-clip seats into a groove in the side gear. The cross pin shaft is then installed, preventing the axle from moving inward and the C-clip from falling out.

Center Axle Disconnect

A device that disconnect one front axle shaft from the differential via a splined, sliding collar. This system only works with open differentials and it still allows the differential side and spider gears in the differential to rotate.

Crawl Ratio

AKA Final Drive Ratio. The maximum multiplied lowest gear ratio, to include first gear, transfer case low range, and axle ratio. If there is another gearing device that ratio is also added.

Cross Pin Shaft

This is a hardened shaft which installs into the case and keeps the spiders securely in place. In semi-float applications, it also prevents the axles from sliding inward into the carrier case.

Crown Gear

This is a commonly used term for the ring gear.

Crush Sleeve

This is a collapsible spacer which installs on the pinion and preloads the pinion bearings. Crush sleeves are not reusable and must be replaced if the pinion nut is loosened or removed.

Cryogenic Treatments

This is the opposite of heat treating and it’s done to a part that is generally finished, such as a complete axle shaft. The part is cooled by normal refrigeration to about 100 degrees below zero, Fahrenheit, then further cooled by nitrogen gas to about 300 degrees below zero. The timing and length of the treatment is precisely controlled according to the material and the end result desired and, after cooling, the part is heated to over 300 degrees then allowed to cool naturally in the air. The advantages are that it relieves internal stress in parts and tightens the grain structure. The treated part is much more fatigue resistant, though only very slightly stronger. Dimensional changes are usually miniscule or non-existent.

Declutching

Usually, pressing in the clutch to uncouple the engine from the drivetrain. It can also refer to the neutral position of items like the transfer case, PTOs, winches, CADs, and other items that use a mechanical disconnect.

Dropout

A dropout is a type of differential housing in which the gear case assembly can be removed from the housing. Other differential housings simply have inspection covers which are removed, allowing you to work on the differential inside the housing. The advantage of a drop out style housing is the ability to work on the differential on a bench.

Ductile

A property of metal referring to its ability to deform without rupture. Closely related to the terms “malleable” and “malleability”. Used to describe the malleability of a finished part, where certain amounts of that quality are desirable.

Elastic Limit

The point at which the material cannot return to its original shape.

Elasticity

The property of metal that allows it to be deformed (strained) but return to its original shape.

Emulsified

Referring to a situation where water and oil are mixed, resulting in a light brown substance in the axle housing. This should be removed immediately as it can create acids that can cause corrosion.

Endurance Limit

The amount of stress below which a part will survive a very high number of stress cycles without failure. For example: an axle shaft may survive 1,000,000 cycles of being twisted one degree; 200,000 cycles of being twisted five degrees; but only 50 cycles of being twisted ten degrees.

Fluid Friction

Friction, or drag, that comes from the viscosity of fluids. Heavier oils will create more drag which will sap horsepower.

Forging

This is divided into two categories, hot forging and cold forging. Hot forging is done when the metal is red hot and soft. Cold forging is done with the metal no hotter than 1/3 its melting point. In both cases, the metal is worked under high pressure (usually a giant press or hammering machine) to tighten the grain structure of the steel and make it stronger. Often the steel is formed into a particular shape at the same time.

Full Floating Axle

A full floating axle is a housing type in which the axle does not carry the load of the vehicle and does not have the wheel bolt directly to it. Full floating axles bolt to a spindle, which ultimately drives the wheels.

GCVWR

Gross Combined Vehicle Weight Rating. The maximum weight for a vehicle, its cargo, and its towed load.

Hardness

A property of metal, relating to the strength of the material.

High Gearing

Numerically low gear ratios. Generally speaking, ratios from 2.50:1 to 3.54:1 are considered “high”.

High Pinion

High pinion refers to a housing where the pinion enters above the centerline. This is also called a “reverse-rotation” housing. High pinion housings are only found in the fronts of some factory vehicles.

I.F.S.

This stands for “Independent Front Suspension”, a housing which allows the left and right axles to move up and down separately from each other.

I.R.S.

This stands for “Independent Rear Suspension”, a housing which allows the left and right axles to move up and down separately from each other.

Induction Hardening

This is a method of case hardening. It’s a common hardening method for axles because it’s fast and can be localized and controlled. Typically, electromagnetic coils are used and the part is heated to a precise temperature and for a precise time, sometimes in several cycles, then instantly cooled. The beauty of induction hardening is that the depth of the hardening can be very precisely controlled.

Limited Slip

A limited slip differential is a type of carrier case where the spider gears are preloaded and prevented from turning easily via a friction surface. Once enough power has been applied to the spiders, which is called a breakaway point, the spiders can turn. The preload and friction can sometimes cause noise or “chatter” when negotiating a turn.

Locker

A locker is a mechanical ratcheting unit that drives both tires when under power. A locker does allow one side to disengage when turning, often resulting in a “banging” or “clunking” sound.

Low Gearing

Numerically high gear ratios. Generally speaking, ratios from 4.10:1 to about 6.13:1 are considered low gears. Lower than 4.88:1 is “super” low while ratios from 3.54:1 to 4.10:1 are regarded as moderately low.

Malleable

The property of a material to bend or twist without rupture, usually referring to steel in an unfinished state.

Nitriding

Nitriding is a surface treatment that case hardens metal. The part is heated in a closed environment with a nitrogenous material, usually ammonia. The surface absorbs nitrogen and becomes harder. Much harder surfaces can be obtained than with other case hardening processes.

Normalized

Steel that is reheated to its critical range temperature, then cooled in still, open air. The process makes the piece uniform in strength and relieves stresses. Without heat treatment, formed metal will try to go back to its original shape.

Pinion

The input drive for the differential, a shaft with a small gear attached. The “spider” gears in an open differential are also called pinions.

Pinion Angle

The angle of the differential input pinion in relation to the horizontal position.

Pinion Gear

The pinion gear turns the pinion gear, transferring power from the driveline.

Pinion Preload

Pinion preload is the amount of resistance or “drag” measured when attempting to turn the pinion without the carrier installed.

Pinion Yoke

The pinion yoke attaches to the pinion gear to the driveshaft.

Posi/Positraction

This is commonly used term for a limited slip.

Pour Point

The lowest temperature at which a specific gear oil will pour.

Ring Gear

The ring gear bolts to the carrier and is driven by the pinion gear.

Semi-Float Axle

A semi-floating axle rides on an outer bearing, thus supporting the weight of the vehicle. In semi-floating applications the wheel is bolted directly to the axle.

Shim

A shim is a thin piece of metal that is used to either adjust the fitment of other parts or take up a gap.

Side Gear

The side gear fits into the housing and has splines on the inside which accepts the axle splines. Side gears are part of the spider gear set.

Spider Gear

The spider gears are a set of four to six gears which install into the carrier. The rotation of the spider gears allows one axle to spin faster or slower than the other.

Spool

A device that connects the two axles directly to the ring gear.

Strain

The deformation of a part caused by stress.

Stress

The forces acting on a part that will cause strain. The ability of a material to resist stress is expressed as “yield” or “tensile strength”.

Tensile Strength

The point at which the material ruptures (breaks). Often expressed in psi.

Third Member

A third member is a type of differential housing in which the gear case assembly can be removed from the housing. Other differential housings simply have inspection covers which are removed, allowing you to work on the differential inside the housing. The advantage of a drop out style housing is the ability to work on the differential on a bench.

Thru Hardening

The entire part is hardened to a uniform standard by heating and cooling according to a precise timetable and temperature chart.

Torque

The rotational force measured in pounds-feet or Newton-meters.

Torque Converter

A fluid coupling/clutch used in automatic transmissions. It consists of three main parts: the impeller, which is driven by the engine; the turbine, which is connected directly to the transmission; and this stator, which lies between the two.

Toughness

A metal property that describes the amount of energy the material can absorb before failing in some way.

Upset Forging

This is a method where flanges are forged onto the end of metal bar stock that will become an axle. Typically, the bar is near its melting point. It goes into a press, the shaft is clamped in a die at a particular point and the portion above the die is tooled into the desired shape. It not only produces the flange but hardens it to a desirable point. Often, the flange area receives no further hardening.

Yield Point/Yield Strength

The point where the material takes on a noticeable, permanent deformation. Yield strength is the rated ability of the material to resist this permanent deformation, typically expressed in psi.