Why bending moments are critical in conveyor pulleys
Drive and bend pulleys in conveyor systems for mining, raw materials or plant engineering are among the most demanding applications for shaft-hub connections. Belt tension bends the shaft, and because the load varies with the material carried, the deflection changes constantly. The connection between shaft and pulley end disc is therefore subjected not only to torque but also to alternating bending moments, often combined with axial forces.
If the connection is not designed for this, micro-movements occur in the joint. The consequences range from fretting corrosion, deformed end discs and axial drift of the pulley to shaft fracture. Such damage often only shows after months and then causes unplanned downtime.
How a conical clamping element works
A conical clamping element, also called a locking assembly, consists of rings with tapered surfaces. When the clamping screws are tightened, the tapered surfaces slide against each other and generate high radial pressure on the shaft and hub bore. Force is transmitted by friction, without a key and therefore without a notch in the shaft.
- Backlash-free connection, even with reversing rotation
- No keyway in the shaft, hence higher fatigue strength
- Infinitely adjustable hub position, axially and circumferentially
- Releasable via jacking threads, even after long service
- Self-centring with good concentricity in suitable designs
It is not torque alone but the combination of torque, bending moment and axial force that determines the right size.
Design: the key parameters
| Parameter | What matters |
|---|---|
| Transmissible torque | With a safety factor for start-up, braking and blocking |
| Permissible bending moment | Explicitly stated in the data sheet for pulleys, not just the torque |
| Axial force | Simultaneous forces reduce the reserve available for torque |
| Pressure on the hub | Hub wall thickness and material must absorb the pressure without permanent deformation |
| Fits and surfaces | Tolerances and roughness of shaft and bore as specified by the manufacturer |
| Environment | Dust, moisture and temperature affect corrosion protection and material choice |
Specialist manufacturers offer series whose design, materials and surface finish are specifically tailored to high bending moments. RINGSPANN, for example, describes its RLK series, according to the manufacturer, as developed for conveyor pulleys, based on the specifications of well-known pulley makers and backed by quality assurance that includes concentricity optimisation. With every supplier, check whether the bending moments for your load case have been verified by calculation.
Typical damage patterns and how to avoid them
| Damage | Common cause | Countermeasure |
|---|---|---|
| Fretting corrosion on the shaft | Micro-movements due to insufficient pressure | Size for the bending moment, observe tightening torque |
| Deformed end disc | Hub too weak for the contact pressure | Check hub geometry and material |
| Axial drift of the pulley | Insufficient reserve under combined load | Include axial forces in the design |
| Difficult disassembly | Corrosion, no jacking threads | Choose a suitable design, pay attention to corrosion protection |
| Shaft fracture | Notch effect, alternating bending, fretting | Properly designed friction connection without keyway |
Assembly and disassembly
- Clean shaft and bore and check for damage.
- Prepare the lubrication state of contact surfaces and screws exactly as specified; the wrong lubricant changes the transmissible torque.
- Insert and align the clamping element.
- Tighten the screws crosswise in several passes with a torque wrench to the specified torque.
- Check the tightening torque after a defined operating period if the manufacturer requires it.
- To release, relieve the screws and separate the rings using the jacking threads.
The assembly instructions are part of the design
A clamping element's load capacity only applies if it is assembled correctly. Give the manufacturer's instructions to the assembly team, record tightening torques and train maintenance staff before the first replacement is due.
Frequently asked questions
When are conical clamping elements the right choice?
Why is a keyed connection often not enough?
What data does the manufacturer need for sizing?
Can I simply replace an existing element with a larger size?
How often should the connection be checked?
Considering load case, hub and assembly together lays the foundation for long, trouble-free operation.