Selecting a radial clearance (Preload) for THK LM guide

Since the radial clearance of an THK LM Guide greatly affects the running accuracy, load carrying capacity and rigidity of the LM Guide, it is important to select an appropriate clearance according to the application. In general, selecting a negative clearance (i.e., a preload* is applied) while taking into account possible vibrations and impact generated from reciprocating motion favorably aff ects the service life and the accuracy.

The clearances of all THK LM Guide models (except model HR, GSR and GSR-R, which are separate types) are adjusted as specified before shipment, and therefore they do not need further preload adjustment.

* Preload is an internal load applied to the rolling elements (balls, rollers, etc.) of an THK LM block in advance in order to increase its rigidity.

Types of Radial Clearance
  Normal Clearance Clearance C1 (Light Preload) Clearance C0 (Medium Preload)
Condition
  • The loading direction is fixed, impact and vibrations are minimal and 2 rails are installed in parallel.
  • Very high precision is not required, and the sliding resistance must be as low as possible.
  • An overhang load or moment load is applied.
  • LM Guide is used in a singlerail confi guration.
  • Light load and high accuracy are required.
  • High rigidity is required and vibrations and impact are applied.
  • Heavy-cutting machine tool
Examples of applications
  • Beam-welding machine
  • Book-binding machin
  • Automatic packaging machine
  • XY axes of general industrial machinery
  • Automatic sash-manufacturing machine
  • Welding machine
  • Flame cutting machine
  • Tool changer
  • Various kinds of material feeder
  • Grinding machine table feed axis
  • Automatic coating machine
  • Industrial robot
  • various kinds of material high speed feeder
  • NC drilling machine
  • Vertical axis of general industrial machinery
  • Printed circuit board drilling machine
  • Electric discharge machine
  • Measuring instrument
  • Precision XY table
  • Machining center
  • NC lathe
  • Grinding stone feed axis of grinding machine
  • Milling machine
  • Vertical/horizontal boring machine
  • Tool rest guide
  • Vertical axis of machine tool

 

Service Life with a Preload Considered

When using an THK LM Guide under a medium preload (clearance C0), it is necessary to calculate the service life while taking into account the magnitude of the preload.

Rigidity

When a load is applied to an THK LM Guide, the bearings and LM block will elastically deform within the allowable load range. The ratio of displacement to applied load is referred to as “rigidity.” The radial internal clearance (preload) for the LM Guide can be specified in order to reduce displacement.

By using balls larger than the width of the race, they will naturally deform elastically as they roll, allowing the load to be maintained for longer while limiting displacement in the LM Guide.

The effect of the preload can be up to 2.8 times greater than the size of the preload itself. If that level is exceeded, the preload is released and the effect of the preload is lost.

When a preloaded LM Guide takes an external load, the displacement will be linear. The level of displacement will be approximately half that of an LM Guide with no preload.

The preload, in addition to reducing displacement, helps prevent premature failure due to vibration and impact/shock.

Rigidity Data