Spindle Fretting Explained

Fretting of Lucas HBM Spindles

What is this document?

The problem of 'Fretting' or 'Fretting Corrosion' on the Spindles of Lucas HBM's is a question that is frequently asked in telephone service calls to Lucas.  This document will attempt to describe what the author knows of the phenomena from many years of conversations with Lucas engineering and service personnel.

 

What is 'fretting' and what does it look like?

Fretting (AKA fretting corrosion) is a degradation of the outer surface of the Spindle and the inner surface of the Spindle Sleeve Liner Bushings that is usually spotted as one or more discolored 'patches' or 'bands' on the O.D. of the Spindle.  The affected area may be either in the form of circular 'rings' around the Spindle O.D. that appear to be either a brown, black or red stain, or a patch of rust or it may appear in non-symmetrical 'patches' randomly located on the Spindle.  It may also appear that the Spindle has been partially plated with either brass or copper.

The metal degradation happens when the 'high points' of the surface finish of the Spindle rub rapidly over the 'high points' of the surface finish of the Spindle Sleeve Liner Bushings and the lubricating oil film does not prevent the metal from actually coming into physical contact.  Under this condition, the 'high points' are 'knocked off' and form loose debris in the joint.  Some believe that the high points actually weld together momentarily (on a microscopic level) and that the material is removed by being 'ripped' wholesale from the tip of the 'high point'.  With a sufficient quantity of proper lubrication in the interface between the two parts, the 'high points' of the surface finish never actually touch each other and fretting never begins.

Fretting is a problem that must be addressed and corrected. While the initial stages are simply discoloration of the Spindle O.D. and the intermediate stages involve what appear to be 'pin holes' through the outer layer of the Spindle: the condition can progress rapidly to the final stages in which LARGE chunks of the surface of the Spindle are removed. When this occurs, the Spindle is SCRAP and no repairs are possible except to replace the Spindle outright, replace the Spindle Sleeve Liner Bushings and perform a complete Spindle Sleeve rebuild.

 

The conditions that foster Spindle fretting are:

  1. Small amplitude rotational motion between the Spindle and the Spindle Sleeve Liner Bushings as the Spindle 'works' during cutting operations by the amount of final drive key clearance (usually at a relatively high frequency).
     
  2. A close fit between the Spindle O.D. and the Spindle Sleeve Liner Bushings.
     
  3. A condition of 'inadequate lubrication' FOR THE OPERATING CONDITIONS.  This is NOT to say that the lubrication is inadequate - simply that it is inadequate for the conditions.  In other words: if the conditions are slightly different - the same lubrication may be completely sufficient.  Don't ‘jump the gun' and blame the lubrication - it may not be what is really wrong!

     

Many customers first experience with fretting is to notice it on the Spindle of a new machine or on the Spindle of a rebuilt Spindle & Spindle Sleeve.  This is occasioned by the closeness of the fit on a new machine or a rebuilt spindle.  I have never seen nor heard of fretting occurring on a worn Spindle.  The relatively large clearances of a worn Spindle and worn Spindle Sleeve Liner Bushings allow enough lubrication to enter the area between the two parts to effectively prevent fretting.

 

What can be done?

From the causative effect of fretting, it can be seen that preventing fretting is a simple matter of removing one or more of conditions that allow fretting to happen, however, some of the 'cures' are simply not practical.

  1. If the Spindle to Spindle Sleeve Liner Bushing clearance is increased to allow oil to enter more freely, certainly fretting will not occur, however, the excessive clearance will tend to make the machine 'chatter' in milling cuts and the machine will not be capable of as much accuracy in cutting than if the fit is close.
     
  2. Stopping the ability of the Spindle to have small amplitude rotational motion between the Spindle and the Spindle Sleeve Liner Bushings could be done by simply ‘tightening up' the clearance of the main Spindle driving key, however, to actually prevent rotation & subsequent fretting would require 'zero' clearance - which would effectively stop the Spindle from being fed - effectively turning the machine into a 'fixed Spindle' design.
     
  3. Reducing the tendency of the Spindle to ‘move’ in the key clearance by being certain to keep the cutter fully ‘loaded’ into the cut can provide a small amount of relief.  For example, fretting is seldom seen in a Spindle that is used primarily for 'constant load' cutting operations like drilling or boring while it is much more frequently seen in machines that are used primarily for milling due to the constantly changing load situation presented by milling cutters.
     
  4. The only certain means of preventing fretting, however, is to be certain that a lubricating film is ALWAYS present between the Spindle O.D. and the Liner Bushing I.D.
     
  5. While this seems simple enough, there are frequently other considerations that cause this objective to be more difficult to obtain than are obvious at first glance.

    A: There is no 'positive' means of injecting lubrication into the joint.  Lubrication enters the area only by being 'dragged' in as the Spindle is moved back-and-forth in a feeding direction.  Essentially, if you don't feed the Spindle every so often - very little to no NEW lubrication ever enters the joint.

    B: Being a horizontal Spindle arrangement, any lubrication that gets into the area between the Spindle and the Sleeve Liner Bushings is slowly drained from the joint by the simple effects of gravity.  There has never been any practical means to seal lubrication between the Spindle and the liner bushings.

    C: Any amount of bearing heat in the Spindle Sleeve carrier bearings will 'shrink' the 'fit' between the Spindle O.D. and the I.D. of the liner bushings in the area directly under the bearings.  This reduced 'fit' has been known to actually 'squeeze' the fit down into a metal-to-metal, 'zero clearance fit' which (obviously) squeezes out any oil that might have been in the joint.  Even if the 'fit' is simply reduced and not eliminated, the overly close fit tends to accelerate fretting problems.

Fretting that is related to 'fit' problems occasioned by bearing-heat generated oil exclusion are usually seen as 'bands' on the Spindle - the irregular 'patch' type of fretting is usually more indicative of uniformly tight fits and are more related to general lubrication exclusion. Fretting in 'bands' is also seen if the alignment of the Head Extension is improper and the Spindle Feed thrust bearings are applying 'side-loading' to the rear end of the Spindle. In this situation, the fretting is observed at the extreme rear end of the rearmost Spindle Sleeve Liner Bushing instead of in the area 'under' the Spindle Sleeve main bearings.

 

So much for the theoretical: what can really be done to arrest fretting?

The most practical and certain method of preventing general fretting (except the bearing-heat oil exclusion type) is to regularly feed the spindle 'full-stroke' to carry oil into the joint between the Spindle and Spindle Sleeve Liner Bushings.  How often this needs to be done will need to be determined separately for the specific machine involved.  A machine that operates at generally higher spindle speeds will generally 'shrink' into a closer fit with the Spindle O.D. (allowing generally less lubrication film thickness in the joint) and will need to be stroked more frequently than a machine that is generally operated at slower speeds with subsequently less heating.

The type of fretting produced by bearing-heat oil exclusion MUST begin by reducing the bearing heat.  In a similar manner, if the fretting pattern shows that the Head Extension is out of alignment with the Spindle and Spindle Sleeve, the misalignment MUST be corrected.  Only after localized heating or alignment problems of the head extension are corrected will the maintenance of the lubricating film produce the desired result.

When fretting is seen on the Spindle O.D. it should be removed by hand polishing the Spindle O.D. with 'crocus cloth' in long, sweeping strokes against a slowly rotating Spindle.  This will produce a 'barber pole' effect on the Spindle appearance on the O.D. of the Spindle that will tend to exclude dirt from entering the joint.  Avoid holding the 'crocus cloth' in one location as the Spindle rotates or using high speeds.  That will tend to produce bands of micro grooving that will, in turn, tend to draw contamination into the joint between the Spindle and the Spindle Sleeve Liner Bushings and will greatly increase the chances of seizing the Spindle.  After the polishing operation is complete, hand wipe the residue from the Spindle - apply oil to the exposed section of the Spindle - and stroke the spindle in-and-out several times to carry it into the joint. If the fretting is attended to promptly, this may be all that is required, however, always recall that there are 'matching' marks INSIDE the Spindle Sleeve Liner Bushings that may require the removal of the Spindle and subsequent polishing.

There have been reports that a program of regularly hand-applied Molybdenum Disulfide bearing oils or greases to the O.D. of the Spindle will lower the chance of fretting by providing a lubrication that cannot be 'drained away'.  This author cannot recall actually having seen this done, but I have always heard good reviews of its effectiveness.

See also - Machinery’s Handbook “Fretting”.

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