Transmission Shafts: Types, Components, and Mounting
Item 6: Transmission Shafts
1. Shafts
Shafts are responsible for transmitting torque from the gearbox to the transfer case or differential. They are used in vehicles to transmit torque from front to rear or vice versa. In front-engine, front-wheel-drive vehicles, they are not necessary because power transmission is done directly by semi-shafts. 4×4 vehicles have two drive shafts: a short one to the front and a longer one to the rear. The connection between them is not rigid and is made using universal joints that allow for reciprocation of the assembly and absorb varying degrees of tilt in the axles during suspension travel.
Constitution
Made of high-quality steel, with a larger diameter in the center, and perfectly balanced. A robust element subjected to torque and fatigue, made as light as possible. It typically consists of two Cardan joints and a body spanning the tree to absorb the up-and-down movement of the rear axle. It has a critical vibration speed that must be avoided. This speed depends on the distance between joints and the stiffness of the tube used.
Universal Joint
Used for steering and transmission shafts, usually mounted in pairs. They can transmit the motor torque while allowing angular displacement of up to 25°. A drawback is the angular velocity variation, which increases with the angle. When two joints are mounted, the speed variation of one compensates for the other, maintaining constant output speeds. They consist of yokes connected by a cross-shaft (spider) and cups mounted on needle bearings.
2. Semi-Shafts (Drive Shafts)
Semi-shafts transmit torque from the differential’s planet gears to the wheels. Their design depends on the type of propulsion and suspension used.
For Independent Suspension
- Ball Joint Boots (Rzeppa): Use six balls held by a cage, moving within six grooves in rings that form the other half of the joint. It allows for longitudinal sliding, unlike a Cardan joint.
- Homokinetic Slider: Compensates for axial movement caused by the shortening of the shaft when the wheel moves up and down in any position of the transmission. It consists of a tripod with three rollers and needle bearings that fit into three slots in the cylindrical housing of the transmission.
- Homokinetic Double Cardan: Uses two universal joints connected by a double-wishbone piece. The alternating spin produced by one is offset by the other. It cannot slip, so it cannot be used alone when the drive shaft has independent suspension.
Semi-Shafts for Drive Axles (Bearings)
Used in 4x4s, trucks, and industrial vehicles. Forged and hot-stamped, hardened by tempering and surface hardening. It is a one-piece semi-shaft without a CV joint, mounted on a rigid axle. One end engages with splines in the differential’s planet gear, and the other end connects to the wheel, requiring two bearings.
3. Mounting Systems
- Semi-Floating: In light vehicles, one end of the shaft rests on the planet gear and its bearing, and the other end rests on the axle tube with a ball bearing. The vehicle’s weight rests on the shaft.
- 3/4 Floating and Full Floating: Very similar, differing in the number of bearings mounted on the stub end of the axle: one for 3/4 floating and two for full floating. The vehicle’s weight does not rest on the shaft, which does not support bending stresses. Used in trucks and industrial vehicles.