English
Türkçe
An electric motor does not only produce power and speed; it also carries a mechanical load through its shaft. A belt-pulley drive applies a side (radial) force to the shaft, while a coupling or gear connection can create a thrust (axial) force along the axis. These forces are carried by the motor's bearings, and every motor has a specific permissible radial and axial load limit depending on shaft diameter and bearing arrangement. When this limit is exceeded, bearing life shortens rapidly, the shaft can bend, vibration and heating rise, and early failure becomes inevitable. Correct motor selection requires accounting not only for power but for the mechanical loads coming onto the shaft. In this article we cover where radial and axial load come from, what permissible load curves mean, how bearing life (L10) relates to these loads, the role of shaft diameter, and how to make the right connection and motor choice, in engineering terms and with a buying decision in mind.
What Are Radial and Axial Load?
Forces acting on the shaft are classified in two main directions. Radial load is the force acting perpendicular to the shaft axis, that is sideways. Its most typical source is the belt-pulley drive; belt tension constantly pulls the shaft end to the side. Axial load is the thrust force acting along the shaft axis, forward or backward. In coupled connections, misalignment, the thrust produced by helical gears in gearboxes, or the rotor weight in vertically mounted motors can create axial load.
These two loads stress different bearing regions and different mechanisms. Radial load primarily stresses the ball-race contact surface of the bearing and the shaft against bending; excessive radial load causes shaft deflection (bending) and early bearing fatigue. Axial load pushes the bearing along the axis; standard ball bearings can carry limited axial load, and high axial load requires a special bearing arrangement. In correct motor selection both loads must be assessed separately.
Source of Radial Load: Belt-Pulley Tension
Belt-pulley drive is the most common radial load source a motor faces. For the belt to transmit power it must have a certain pretension; this tension reflects as a constant side force at the shaft end. The magnitude of the radial load depends on several factors:
- Pulley diameter: A small pulley requires higher belt force to transmit the same torque, increasing radial load. A large pulley reduces radial load.
- Transmitted power and torque: As power rises, the required belt force and therefore radial load rise.
- Belt type and count: V-belt, flat belt or timing belt create different tension requirements.
- Point of force application: The closer the force is to the shaft end, the longer the moment arm on the bearing and the higher the load.
When selecting a motor for a belt-pulley application, therefore, not only power and speed but also pulley diameter and belt arrangement must be considered. Transmitting high power with a small pulley can push the radial load on the shaft above the motor's limit.
Permissible Load Curves and the Role of Shaft Diameter
For each frame size the manufacturer gives the permissible radial load as a curve (load diagram). The basic logic of this curve is: the permissible radial load depends on the distance of the force from the shaft end (the x dimension). As the force approaches the shaft end the moment arm lengthens and the permissible load decreases; as the force approaches the frame (bearing) the permissible load increases. Also, as speed increases the permissible load generally decreases for bearing life, because at high speed the same load produces more fatigue cycles.
Shaft diameter is decisive in this equation. A larger frame size means a thicker shaft and a larger bearing; this provides both higher resistance to bending and a higher permissible load. The table below conceptually summarizes how load capacity increases as shaft diameter and frame size grow.
| Frame Size (example) | Typical Shaft Diameter | Radial Load Capacity | Axial Load Capacity | Typical Application |
|---|---|---|---|---|
| Small (90-112) | 24-28 mm | Low | Low | Light fan, pump |
| Medium (132-160) | 38-42 mm | Medium | Medium | Belt drive, conveyor |
| Large (180-225) | 48-60 mm | High | High | Heavy belt drive, crusher |
| Very large (250+) | 65 mm and above | Very high | High | Heavy process, large drive |
The values in the table are conceptual and vary by manufacturer; the aim is to show the direct proportion between shaft diameter and load capacity. If a borderline radial load is involved, moving up a frame size (a thicker shaft) is often the safest solution.
Bearing Life (L10) and Load Relationship
Bearing life is expressed in engineering by the L10 life: the time (in hours or revolutions) during which 90 percent of a group of identical bearings is expected to operate without failure. L10 life has a strong inverse relationship with the applied load. In ball bearings, life is inversely proportional to roughly the cube of the load; that is, if the load doubles, the life falls to about one-eighth. This strong relationship explains why the load limit must be taken seriously: even a small overload can drastically shorten life.
- As load rises: L10 life decreases rapidly (cubically).
- As speed rises: More cycles occur in the same time; life in hours shortens.
- Correct lubrication: Grease type and lubrication interval are critical for the calculated L10 life to actually hold.
- Alignment: Poor alignment creates extra loads not accounted for and shortens life below expectation.
In motor selection, therefore, the target bearing life (e.g. a long L10 for continuous process) should be set and the applied radial/axial load assessed against this target. A borderline load means frequent bearing replacement in the long term even if it works in the short term.
Correct Connection and Motor Selection
There are two ways to keep the load on the shaft under control: reduce the load or choose a higher-capacity motor. In practice the right solution is usually a balance of the two:
- Increase pulley diameter: Where possible, using a larger pulley directly reduces radial load.
- Bring the force closer to the bearing: Positioning the pulley near the frame rather than at the shaft end shortens the moment arm.
- Alignment in coupled connections: A well-aligned flexible coupling minimizes extra axial and radial loads.
- A larger frame size: For borderline loads, choosing a motor with a thicker shaft is the safest route.
- External bearing (pillow block): For very high radial loads, the load may need to be transferred to a separate external bearing off the motor shaft.
Correct motor selection begins with clearly defining the application's load profile (radial, axial, speed, target life). When this information is given at the time of ordering, both the correct frame size and, if needed, a special bearing arrangement can be chosen upfront.
Sources of Axial Load and Bearing Arrangement
Axial load, though it draws less attention than radial load, is a critical limitation in many applications. Its main sources are: axial misalignment in coupled connections, the axial thrust produced by helical or bevel gearboxes, hydraulic axial force in centrifugal pumps, air thrust in fans, and the weight of the rotor plus connected equipment in vertically mounted motors. These forces push the shaft along the axis and stress the bearing. A standard motor usually has a fixed (locating) bearing at one end and a free (floating) bearing at the other; this arrangement allows shaft expansion while carrying limited axial load.
When high axial load is involved, the standard arrangement may be insufficient. In that case a bearing type suited to axial load (e.g. an angular contact bearing, a matched bearing pair or a roller bearing) must be selected. The direction of the axial load also matters; a continuous one-directional thrust and a variable-direction thrust require different bearing arrangements. The magnitude and direction of the axial load should therefore be clear before ordering. A wrong assumption leads to the motor failing at the bearing far sooner than expected.
Vertical Mounting and the Effect of Orientation
In vertically mounted motors (e.g. V1 shaft-down position), the weight of the rotor and connected equipment bears directly on the bearing as an axial load. This creates a very different load picture than horizontal mounting and usually requires a special bearing arrangement suited to axial load. Vertical mounting also has a different oil seal and lubrication arrangement; in the shaft-down position, extra measures are needed for sealing and grease retention. Mounting orientation is thus a parameter that directly affects not only the mechanical connection but also bearing selection and the load limit. When selecting a motor for a vertical application, the mounting code (V1, V5, etc.) and the axial load should be assessed together upfront.
Signs of Overload
Signs that the radial or axial load on the shaft exceeds the limit can be noticed before the damage becomes permanent:
- Abnormal and increasing heating in the bearing region.
- Increasing vibration and noise during operation; especially a regular, speed-related sound.
- Grease leakage, darkening of grease color or grease containing metallic particles.
- Deflection (bending) or runout at the shaft end measurable by eye or with a dial indicator.
- Accelerated wear on the coupling or pulley, loss of alignment.
When one of these signs appears, the application's load profile should be recalculated; pulley diameter, force position and motor frame size should be reviewed. Early intervention protects both the bearing and the shaft and prevents unplanned downtime.
Frequently Asked Questions
Why does the motor shaft matter in a belt-pulley application?
Belt tension applies a constant radial (side) load to the shaft. This load must stay below a certain limit set by shaft diameter and bearing capacity. Transmitting high power with a small pulley increases radial load; in that case a larger pulley or a motor with a thicker shaft (a larger frame size) is needed. Otherwise bearing life shortens and the shaft can bend.
How much axial load can a standard motor carry?
Standard ball-bearing motors can carry a limited axial load; this limit depends on frame size and mounting position. If high axial load is involved (e.g. vertical mounting or helical gear thrust), a special bearing arrangement or a motor suited to axial load is needed. The direction and magnitude of the axial load should be clarified before ordering.
What most affects bearing life?
The applied load is the factor that most strongly affects life, because L10 life is inversely proportional to roughly the cube of the load. Speed, lubrication quality and alignment are also decisive. Keeping the load below the limit, lubricating correctly and aligning well ensure the calculated bearing life actually holds.
Choose the Right Motor with Fast Delivery from Stock
Share your application's shaft load profile (radial/axial force, pulley diameter, speed, target life) with us. As HEM Motor, with manufacturer stock advantage and fast delivery, let us determine together the most suitable motor with the correct frame size and, if needed, a special bearing option. You can review our articles on coupling selection and shaft alignment, speed adjustment with pulley-belt, bearing greasing, shaft diameter, key and coupling and vertical mounting selection, then request a quote.
