Do Ball Screws Require Special Mounting Fixtures?

Most ball screws do not require a special-purpose mounting fixture. A standard installation normally uses correctly matched bearing support units, a ball nut housing or bracket, a motor coupling and accurately machined mounting surfaces. Custom fixtures are needed only when the machine structure, available space, screw size or alignment procedure cannot use standard components.

The important distinction is between a mounting fixture and the components required to support and connect the ball screw. Support units and nut housings are normal parts of a ball screw assembly; they should not automatically be described as special fixtures.

What Components Are Normally Needed?

A rotating-screw ball screw installation usually requires the following parts:

• Fixed-end support unit: locates the screw axially and radially and carries the main thrust load;
• Supported or floating-end unit: provides radial support while allowing the selected mounting arrangement to function correctly;
• Ball nut housing or bracket: connects the nut flange to the moving table or carriage;
• Coupling: connects the machined screw end to a servo or stepper motor;
• Motor bracket: holds the motor in the required position relative to the screw axis;
• Machined machine base: provides stable and aligned mounting surfaces for the supports and nut housing.

Common support-unit combinations include BK/BF, FK/FF and EK/EF. The correct series depends on the ball screw diameter, shaft-end dimensions and machine mounting structure. More details are available in What Is a Ball Screw End Support?

When Are Standard Mounting Components Enough?

Standard components are generally sufficient when:

• The ball screw uses common shaft-end dimensions;
• A standard BK/BF, FK/FF or EK/EF support arrangement fits the machine;
• The nut flange can be connected to a standard or simply machined nut housing;
• The motor and screw can be connected by a suitable flexible coupling;
• The machine base provides flat, rigid and accurately positioned mounting surfaces;
• The stroke, speed, load and precision requirements are within the selected component ratings.

For many CNC routers, automation slides, packaging machines and positioning mechanisms, the mounting structure can be built from standard support units and a machine-specific nut bracket. This does not mean that alignment is unimportant. Standard parts still need accurate positioning and correct assembly.

When Is a Custom Fixture or Bracket Required?

A custom fixture may also be useful during assembly even when the finished machine uses standard supports. For example, a temporary support can prevent a long screw from sagging while its bearings and nut housing are being aligned. The temporary fixture is removed after installation and is not part of the final motion system.

Mounting Arrangement Matters More Than the Word "Fixture"

The ball screw must be supported according to its length, speed, axial load and required rigidity. Four common arrangements are:

• Fixed–free: simple configuration for short, light-load and relatively low-speed applications;
• Fixed–supported: commonly used for general CNC and automation axes;
• Supported–supported: provides radial support at both ends but less axial rigidity than a fixed arrangement;
• Fixed–fixed: provides higher axial rigidity and is often considered for demanding high-speed or precision systems.

The selection should not be based only on convenience. Screw length, critical speed, buckling risk, thermal expansion and bearing preload must also be considered. See Four Standard Ball Screw Mounting Methods for a detailed comparison.

A Custom Fixture Cannot Correct Poor Alignment

A rigid fixture does not automatically create an accurate installation. If the fixed support, supported end and nut housing are not coaxial, tightening all components can force the screw to bend or pull the ball nut sideways.

Possible symptoms include:

• Uneven rotational resistance;
• Higher motor current or starting torque;
• Noise and vibration that change along the stroke;
• Local heating near the nut or support bearings;
• Premature wear of the screw raceway, nut or bearings;
• Reduced positioning consistency.

During installation, the supports and nut housing should initially remain slightly adjustable. After the screw is moved slowly through the working stroke and alignment is confirmed, the mounting bolts can be tightened in the specified sequence. A dial indicator can be used to check shaft runout and the relative position of the supports. For more detail, read How to Align Ball Screw Support Bearings Correctly.

What Information Is Needed Before Designing the Mounting?

Before deciding whether standard components or custom fixtures are necessary, confirm:

• Ball screw diameter, lead, total length and accuracy grade;
• Required stroke, speed, acceleration and axial load;
• Horizontal or vertical installation direction;
• Selected fixed, supported or fixed-fixed arrangement;
• Bearing support series and shaft-end dimensions;
• Ball nut flange dimensions and nut housing interface;
• Motor shaft diameter and coupling size;
• Machine-base dimensions and available installation space.

DLY can supply ball screws with matching shaft-end machining for commonly used BK/BF, FK/FF and EK/EF support units. The final end dimensions should be confirmed from the selected support unit or customer drawing before machining.

← Back to DLY Blog