Differential bevel gear and ring gear

Pinion gears and crown wheels together form the bevel gear pair in the final drive. They change the direction of drive and provide a fixed gear ratio before torque is distributed to the half shafts. Correct matching and adjustment determine noise levels, heat generation and service life. On MAN and Mercedes trucks, buses and vans, replacement therefore involves more than simply fitting gears that match the dimensions.

Always identify parts using the vehicle chassis number, together with the axle code, axle identification plate and part numbers. Visual comparison or selection based solely on the model name is never sufficient. MAN TGS, TGX and Mercedes Actros or Arocs, for example, are available with different axle variants and reduction ratios. On buses and rear-wheel-drive vans, the year of manufacture, axle load and drivetrain configuration can also result in different gear pairs.

Distinguish between spiral bevel gears and hypoid gears, in which the pinion axis is offset relative to the crown wheel axis. Hypoid gears have more sliding contact between the teeth and place specific demands on the lubricating film. These gears are generally made from carburised and hardened alloy steel, providing wear-resistant tooth flanks with a tough core to withstand varying loads.

The ratio is calculated by dividing the number of crown wheel teeth by the number of pinion teeth. Also check the direction of rotation, mounting dimensions, bearing seats and any hub reduction. Where multiple driven axles are linked, their overall axle ratios must match. Engine families such as MAN D26 and Mercedes OM 471 help indicate the load application, but do not determine part selection on their own.

A whining noise during acceleration or engine braking may indicate incorrect tooth contact. Pitting, spalling and scuffing are consistent with fatigue, loss of the lubricating film or overloading. However, a clunk when the load changes can also be caused by worn propshaft universal joints or excessive play elsewhere in the drivetrain. Locate the cause before dismantling.

1. Carry out a controlled road test and record the speed, load and temperature at which the noise occurs. Compare operation under power, coasting and engine braking.
2. Check the axle gear oil level, look for leaks and inspect the axle breather. Examine a drained oil sample and the magnetic plug for metal particles; large fragments require further investigation for damage.
3. Measure backlash with a dial indicator against the crown wheel teeth while holding the pinion stationary. Repeat at several positions around the circumference. Also measure axial runout on the specified reference face.
4. Check the pinion bearings for axial play and measure the rotational torque with a suitable torque gauge under the specified assembly conditions. Seal friction affects this measurement.
5. Apply gear marking compound and assess both tooth flanks while applying resistance. Compare the contact pattern with the manufacturer's specifications for pinion depth and backlash.

Replace damaged pinion gears and crown wheels as a matched set; do not combine unrelated individual gears. Check the tapered roller bearings, bearing fits, crown wheel flange and differential case. Contamination or burrs between mounting faces cause runout and local variations in backlash.

Set the pinion depth and bearing preload first, then adjust the crown wheel position and backlash according to the axle documentation. Use the specified shims and renew collapsible spacers and single-use fasteners where required. Universal backlash values are unsuitable: tolerances and torque settings vary with axle design.

Clean the axle housing thoroughly after tooth breakage and use only gear oil with the specified viscosity and manufacturer approval. After assembly, recheck the tooth contact pattern, free rotation and sealing. A final road test, checking for leaks, noise and temperature, helps detect installation errors early.