A drive shaft is a rotating shaft that transmits rotary motion and torque from a power source (motor, gearbox, transmission) to the machine element that uses it (wheel, pulley, rotor, gear). In cars, the propeller and axle shafts that carry engine power to the wheels are drive shafts; in industry, gearbox output shafts, conveyor pulley shafts, crusher eccentric shafts and rotor shafts are examples.
In this guide we look in detail at what a drive shaft is and what it does, drive shaft types, how drive shaft torque is calculated, shaft materials, the role of dust boots, failure symptoms and the maintenance of drive shafts in mining machinery.
What Does a Drive Shaft Do?
The basic job of a drive shaft is to transmit torque and rotary motion without loss and safely. In addition, it:
- Bridges the distance between the power source and the driven element,
- Tolerates angular offsets and changes of position thanks to universal or constant-velocity joints,
- Accommodates axial length changes through sliding (telescopic) sections,
- Carries elements mounted on it such as gears, pulleys, drums or rotors.
Drive Shaft Types
| Type | Feature | Use |
|---|---|---|
| Straight (rigid) shaft | One piece, supported in bearings | Gearbox, pulley, rotor and eccentric shafts |
| Cardan shaft | Universal joints at each end; allows angular offset | Vehicles, some screen and pump drives |
| Constant-velocity (CV) shaft | CV joints; constant speed transmission even at large angles | Axle shafts of front-wheel-drive vehicles, some mobile plant |
| Telescopic (sliding) shaft | Sliding section whose length can change | Moving connections |
| Hollow shaft | Lighter for the same strength | Long spans, shaft-mounted gearboxes |
| Stepped shaft | Sections of different diameters; seats for bearings, gears and pulleys | Most industrial machines |
What Is Drive Shaft Torque and How Is It Calculated?
Torque is the turning effect of a shaft and is expressed in newton-metres (Nm). The torque a drive shaft transmits is related to the power transmitted and the speed. The practical formula is:
T (Nm) = 9550 × P (kW) / n (rpm)
For example, the nominal torque transmitted by a 75 kW motor shaft running at 1,500 rpm is about 9550 × 75 / 1500 ≈ 477 Nm. When this power is reduced to 50 rpm by a gearbox, the torque on the gearbox output shaft rises to about 14,300 Nm (excluding efficiency losses). This example shows why low-speed shafts must be much thicker and stronger.
Shaft design takes into account not only nominal torque but also starting torque, shock loads (very important in crushers), bending moment and fatigue. Shafts are therefore sized using a service factor and fatigue calculations.
Shaft Materials and Heat Treatment
| Material Group | Feature | Use |
|---|---|---|
| Carbon steels (e.g. C45 grade) | Economical, easy to machine | Light and medium loads |
| Quenched and tempered steels (Cr-Mo, Cr-Ni-Mo alloyed, e.g. 42CrMo4 grade) | High strength and toughness | Heavy loads and shock; crusher and gearbox shafts |
| Case-hardening steels | Hard surface, tough core | Gear shafts |
| Stainless steels | Corrosion resistance | Wet and corrosive environments |
Heat treatments such as quenching and tempering, induction surface hardening and stress relieving determine the shaft’s strength and fatigue life. Grinding shaft surfaces and machining transition radii correctly prevent cracks from starting.
What Are Dust Boots?
CV and universal joints, used especially in vehicles and some mobile plant, are protected by dust boots (rubber gaiters). The boot holds the grease inside the joint and keeps dust, water and mud out. When a dust boot tears:
- Grease is thrown out,
- Dust and water get into the joint,
- Wear, play and noise soon begin in the joint (especially a “clicking” sound when turning),
- If left too long, the whole joint must be replaced.
A “drive shaft dust boot damaged” warning or a visible tear should therefore be dealt with without delay. With early intervention, the joint can be saved by renewing only the boot and grease.
Drive Shaft Failure Symptoms
| Symptom | Possible Cause |
|---|---|
| Vibration (increasing with speed) | Imbalance, bent shaft, worn joint |
| Clicking or knocking | Play in the universal or CV joint, worn key/keyway |
| Grease leakage | Torn dust boot, seal damage |
| Bearing overheating | Misalignment, bent shaft, bearing failure |
| Keyway widening | Loose fit, shock load |
| Shaft breakage | Fatigue, overload, notch effect, corrosion |
Most shafts break not suddenly but through fatigue: a small crack starting at a notch, keyway corner or corrosion point advances a little with each revolution until the shaft finally breaks. “Beach marks” on the fracture surface are the typical sign of this kind of failure.
Drive Shafts in Mining Machinery
| Equipment | Shaft | Critical Point | Guide |
|---|---|---|---|
| Jaw crusher | Eccentric shaft | Bearing seats, shock and fatigue | Eccentric shaft |
| Cone crusher | Countershaft (pinion shaft), main shaft | Pinion contact, bushings | Countershaft and pinion |
| Impact crusher | Rotor main shaft | Balance, bearing seats | Main shaft and bearings |
| Vertical shaft crusher | Rotor shaft (inside the bearing cartridge) | High speed, lubrication | Bearing cartridge |
| Vibrating screen | Eccentric shaft, cardan shaft | Bearings, unbalance weights | Screen spare parts |
| Conveyor | Drive pulley shaft | Locking elements, bearings | Conveyor pulley |
| Screw washer | Screw shaft | Lower bearing, wear | Sand screw washer |
Maintenance and Inspection
- Alignment: Check alignment between the motor, gearbox and driven shaft with a laser tool.
- Vibration monitoring: Imbalance and joint wear are detected early by vibration analysis (predictive maintenance).
- Greasing: Universal joints and sliding sections should be greased periodically.
- Dust boots and seals: Inspect visually on a regular basis.
- Non-destructive testing: Critical crusher and rotor shafts can be checked for cracks by dye penetrant or magnetic particle methods during major overhauls.
- Keys and fits: Measure keyways and fit surfaces at every dismantling.
Repair or New Shaft?
Shafts with worn bearing or seal surfaces can be repaired by methods such as metal spraying, weld build-up and remachining, or fitting a sleeve (repair sleeve). However, weld repair of highly loaded crusher and rotor shafts should be assessed carefully because it can reduce the material’s fatigue strength. Heavily loaded shafts with cracks should generally be replaced. For repair methods using bushes and sleeves, see our shaft bushing guide.
Frequently Asked Questions
What does a drive shaft do?
It transmits torque and rotary motion from the motor or gearbox to wheels, pulleys, rotors or other machine elements.
What is drive shaft torque?
The turning moment the shaft transmits. In practice it is calculated with the formula T (Nm) = 9550 × P (kW) / n (rpm).
How can you tell a drive shaft is failing?
Vibration, clicking or knocking, grease leakage, a torn dust boot and bearing overheating are the main signs.
Is a cardan shaft the same as a drive shaft?
A cardan shaft is a type of drive shaft with universal joints at its ends. Every cardan shaft is a drive shaft, but not every drive shaft is a cardan shaft.
Why do shafts break?
Usually through fatigue; notches, sharp shoulders, keyway corners, corrosion and misalignment initiate fatigue cracks.
Couplings: The Link Between Drive Shafts
Motor, gearbox and machine shafts are usually connected by couplings. A coupling transmits torque while tolerating small misalignments and vibration.
| Coupling Type | Feature | Use |
|---|---|---|
| Flexible (rubber element) coupling | Vibration damping, misalignment tolerance | Pumps, conveyors, fans |
| Gear coupling | High torque, compact | Heavy drives |
| Chain coupling | Simple, economical | Medium loads |
| Rigid coupling | No misalignment tolerance | Perfectly aligned shafts |
| Fluid coupling | Soft start, overload protection | Conveyors, crushers |
| Disc coupling | Maintenance-free, precise | High speeds |
Flexible coupling elements are wear parts and should be replaced periodically. Misalignment is the most common cause of premature failure of coupling elements and bearings.
Information Needed to Manufacture a Shaft
- Technical drawing or sample shaft,
- Material and heat treatment details (if known),
- Dimensions and tolerances of bearing, gear and pulley seats,
- Keyway dimensions,
- Surface hardening or coating requirements,
- The equipment it is used in and the operating conditions.
Glossary
- Torque: Turning moment (Nm).
- Universal joint: A joint that transmits motion between shafts at an angle.
- Constant-velocity (CV) joint: A joint that transmits constant speed at an angle.
- Dust boot: A rubber gaiter that protects the joint.
- Fatigue: Crack initiation and growth under repeated loads.
- Key: A connecting element that transmits torque between shaft and hub.
More Questions
What happens if a drive shaft dust boot tears?
Grease is thrown out and dust and water get into the joint; the joint soon wears and starts to make noise. If caught early, the joint can be saved by replacing only the boot and grease.
Why is shaft balance important?
An unbalanced shaft causes vibration that increases with speed, bearing and seal damage, and fatigue.
CSP Mühendislik Support
CSP Mühendislik manufactures, repairs and supplies crusher eccentric and rotor shafts, conveyor pulley shafts, screw shafts and other drive parts to drawing or sample. You can send your requirements through our service, maintenance and repair services and our spare parts page. For crusher parts see our crusher spare parts page, and for conveyors our belt conveyor spare parts page.
