What Is Spindle Bearing Preload and Why Is It Done?

There are three basic types of preload; each is based on the design and application of the spindle, as determined by the manufacturer. They are:

1. Rigid Preload
   The most common method of preload, rigid preload produces an “interference fit” with the opposing bearing faces when bearings are locked axially. This requires a tremendous amount of skill however, as engineering must take into account any dimensional changes that may be caused by thermal contraction or expansion.
2. Constant Pressure Preload
   Also called spring preload, constant pressure preload produces a series of spring reactions against the bearings. The springs adhere and absorb pressure during any thermal expansion. Variations are minimized in this configuration, but because special care must be given to matching the springs, this preload is more effective on smaller grinding spindles.
3. Variable Preload
   Some newer spindles utilize hydraulic pressure to control the preload. While this method is rare due to its complexity, the spindle bearing preload is varied to keep the bearings in their sweet spot of dynamic load across their operating RPM.

You are probably thinking to yourself: How do I know how much to preload? Luckily for you, you don’t usually have to decide. Leave it to the experts to make their bearing preload calculations to determine the amount for the most precise measurement to streamline the process. It is important to note, however, that based on recent studies, advancements and theories have successfully improved the process — but no bearing preload optimization standards seem to exist.

Bearing ball skidding can be a symptom of an improper preload — usually too little. Essentially, it induces an irregular spindle vibration which causes a severe rise in bearing temperature. The ball skidding effect of the bearing increases from a reduced contact area, or by losing contact completely, when at a certain bearing rotation speed. When ball skidding occurs, friction increases, leading to a significant amount of heat generation. Prevent bearing ball skidding whenever possible.

Simply put: Spindle preload is important because it prevents clearance or looseness that could cause movement. And if the shaft is moving excessively in its housing, the mass will shift off center, causing unwanted inaccuracies and vibration by falling out of alignment.

If you are wondering how to preload a bearing yourself, you probably shouldn’t. Preloading is quite literally an (almost) exact science. The good news is, professionals like those at Northland Tool are happy to help. Technicians will take into account all the variables for a given application, including but not limited to:

• Spindle preload type
• Bearing combination
• Bearing position
• Bearing inner race dimension
• Bearing outer race dimension
• Bearing contact angle
• Shaft seat dimension
• Housing seat dimension
• Bearing MFG ground in preload or clearance
• Maximum RPM
• Axial rigidity requirement
• Radial rigidity requirements
• Heat dissipation design
• Hollow shaft ratio
• Housing wall thickness
• Target static loads

Once all this is calculated and the spindle is assembled, the actual preload can be tested and adjusted if required. This is usually accomplished by axial displacement, which utilizes the relationship between the preload force and the elastic deformations within the bearing arrangement.

Of course, Northland Tool is always available to set the preload on your precision spindle to get production back up and running.

Still unsure of how to measure bearing preload? Contact an experienced Northland Tool technician at (603) 819-4903 today.