Support Units: Four guides: selection, diameter matching, unit cards and bearing loads
Four guides: selection, diameter matching, unit cards and bearing loads
The two ends of a screw do completely different jobs:
| Position | Series | Job |
|---|---|---|
| Fixed end | BK / EK / FK | Carries axial load, locates the screw axially, sets positioning stiffness |
| Supported end | BF / EF / FF | Carries radial load only, allows slight axial float to absorb thermal growth |
Two fixed ends lock thermal growth in and build axial force as the screw warms; two supported ends cannot locate the screw at all. Always one fixed and one supported.
Support units are chosen by screw shaft diameter, not by picking a letter at random:
Each series is stepped by shaft diameter (10, 12, 15, 20, 25, 32, 40 mm and up). Use the shaft diameter as the key.
The bearing is the heart of the unit. At the fixed end it carries axial load, and its dynamic rating sets the unit's axial capacity; at the supported end it mainly carries radial load.
Compare the screw's axial load against the unit rating with a safety margin. At speed, also check the bearing limiting speed — an often-missed step that shows up as heat and noise after a while in service.
Housing bore and shaft end must match. Standards we work to:
If the shaft diameter does not match the bore, machine the shaft end. Shims and force-fitting are not a fix.
Torque the locknut to the value tabulated for the shaft diameter. Too little and it loosens and position drifts; too much and you damage the thread or the bearing.
Precision or long screws are often pre-stretched on installation: an initial axial tension that cancels thermal growth in service and holds positioning accuracy. The amount follows from temperature rise and stroke — overstretching cuts life markedly.
Not recommended. Two fixed ends trap thermal growth, building axial force that shows as heat, higher drag and shorter life. The standard arrangement is one fixed and one supported.
By screw shaft diameter, then by series (BK/BF standard, EK/EF square, FK/FF flanged) and mounting method.
No. The fixed end carries axial load and locates the shaft; the supported end carries radial load and allows axial float. Reversed, positioning fails or the bearing fails early.
Usually the bearing limiting speed has been exceeded, or two fixed ends have trapped thermal growth. Check the limiting speed and confirm a fixed–supported arrangement.