preload ?

[andy55]

Hi group,

please could someone expplain the term preload used in relation to linear rails and bearings.
for example HiWin lists three classes of preload Z0, ZA, and ZB, which correspond to a force between 0 and 0.12C where C is the dynamic load rating.
but, the catalog does not go into details of why/when preload would be good/bad ?
(catalog is at http://www.hiwin.com/catalogs/linear_guideways.pdf)
If I buy a rail+bearings with preload, can I ever take them apart and reassemble them myself or does it have to be done at the factory ?

[NC Cams]

Preload is sort of like "negative clearance".

If you install progressivly larger, oversized balls (higher preload) into the raceway, you:

a. increase stiffness
b. reduce slop/axial/radial clearance
c. increase friction (slightly as it is still rolling friction).
d. increase running accuracy as the shaft can't wobble/orbit within the clearance space which you've removed.

It is always hard to reload a ball equipped linear guide if you take one apart and the balls fall out. The addition of preload only makes it harder. NOTE: in some cases, when you preload a linear guide you alternate ball size (IE: big-small-big-small etc).

This helps the balls roll easier as the small ball acts as an idler. If you don't do it, sticking can be a problem. And one more thing, an "o-size" ball in this case means 0.000050" to 0.000100". Dial calipers WON'T measure that kind of difference and neither will some micrometers.

[veteq]

Hello,
If you are going to mill aluminium or brass or even steel you need the biggest preload.

[NC Cams]

Re: "biggest preload" Hmmmmm. That depends.

High speed linear guides and bearings used in 10K rpm spindles can't run high preloads due to heat build up problems. Thus, these sorts of applications run many light cuts at high speed with low preload bearings. Examples: high speed spindles, high speed lathes, high speed grinders, ultra precision lathes, mirror finish grinders.

Low/moderate speed equipment will run heavier preloads in order to have more rigid bearings. These types of machines will make heavier cuts albeit at lower inch/minute infeeds. SFM is more of a function of finish and the ability of the cutter to cut without galling or tearing the metal. Lathes and milling and drilling machines fit this category.

Low speed machines that hog off the material need the heaviest preload (regardless of the material) simply because the tool loads are high and it is easy to overcome the bearing preload and to get the tool chattering. Rotary mills and other machines that hog off materil fit this criteria.

Thus if one would literally follow the "if you are going to mill aluminum or brass or even steel, you need the biggest prleoad" suggestion, you'd need the biggest preload for pretty much everything which is, in actuality, not the case.

A decent discussion of bearing preloads can be found in the "Precision Rolling Bearings for Machine Tool Spindles" catalog from NSK (Ref Cat #E124)

[veteq]

Hello NC cams,

i didn`t looked at it this way.
Heat build cann be a problem, i`m in the proces of learning a lot to build a hf milling machine for aluminium.
here in the Netherlands the compant thk adviced me to use the ones with the highest preload, because off the vibration, they say they only use this kind for milling machines. Now i dont know it any more.
is it right that hf milling machines designed for aluminium dont have that much vibration?
and is it also right that the hobby machines off us people don`t have that fast feeds in relation with heat build up?
hope you cann explain a bit about it.
The machine i want to make has feeds of max. 70 inch/minute
do i need to use the high preload off thk or will the medium be ok?
kind regards,

Roy Bakker.

[NC Cams]

The proper preload for any spindle bearing is calculated for the spindle maker by the bearing supplier AFTER the spindle engineer supplies specific and highly detailed operation input parameters. Picking a bearing and/or preload base upon generalities generally leads to unsatisfactory performance and/or bearing life.

Factors besides cutter load include bearing packaging (duplex, triplex, quad, roller, angular thrust, bearing size etc) as well as preload method (constant pressure, position preload etc.) must be considered.

Temperature operating ranges as well as lubrication method (oil bath, oil jet, grease, oil/air mist, etc) also must be considered before you even can talk about "how much preload???".

The previously mentioned NSK machine tool bearing catalog provides better insight into what must be considered in specifying how much and what method of preloading should be used.

The referenced catalog provides charts for Light, Med and Heavy preloads for numerous bearings. Even so, specific bearing recommendations are NOT made as special applications must be reviewed by their engineering department for bearing feferrals.

GENERALLY speaking, heavier preloads are used to:

1. prevent the cutting forces from unloading the bearings under heavier cuts.
2. prevent chatter by (see 1 immediately above).

Thus, if you plan on burying a tool into the material to cut it, yes, more preload is better - to a point. However, load is load whether it comes from cutting forces or preload. Thus, you have to look deeper into what's happening, why and how to deal with it properly.

Heavier preloads do tend to reduce life expectancy a bit, again, generally speaking. Thus, you can only use so much preload. For example, a heavy preload for a 20mm bore 7900A5 series duplex bearing would be 245 N while a heavy on a 7200A5 would be 590 N. Same bore size, different OD, much different capacity and thus different preload.

Loads generate heat. High speeds generate higher internal bearing loads (centrifugal forces) as well as frictional heat. Thus, speed also affects preload recommendations (higher speeds use lower preloads and vice versa). Oil

At this point, I hope you can see why it is so difficult to give you a specific answer to your question. You are probably better off to put a detailed list of your operating parameters together and contact a service engineer for a bearing supplier for assistance. They do it for a living and generally do it well.

I no longer have the tools available to calculate bearing loads and fatigue life for machine tool bearings as I did when I worked for a bearing supplier. Hence, I'm only trying to show you that you need to have the proper data for a qualified bearing engineer to help you select the proper bearing size and preload for your application




If you are going to do heavy infeeds and heavy cuts, you may want to use a cylindrical roller bearing instead of a round ball bearing - much more radial capacity. Your options are to use duplex, triplex or quad angular contact ball bearings if you will be running high speeds. Again, more bearings with lighter preload enables you to carry the load at higher speeds.

It takes a VERY well engineered spindle to cut at very high speeds (SFM) and at high infeed rates (inches/min).