What are Permanent Alignment Spacers and Why Should You Consider Them?

By Chuck Summerhill

If you have ever replaced the tapered roller bearings inside of your driven gearboxes, you are most likely aware that the procedure can be tedious.  Typically, people are concerned with setting the proper bearing preload and using the correct stack-up of bearing spacers but fail to consider the spindle shoulder alignment.  Many integrated gearboxes or transmissions incorporate shim packs within the spindle & bearing assembly to allow for lateral adjustment of the spindles for proper shoulder alignment to one another (Figure 2), as well for making fine adjustments to the bearing preload.  However, there are numerous mills on the market that make spindle shoulder alignment difficult.  The trouble with some machines is that when you need to make a lateral adjustment to the spindle, you must loosen the locking nut that you just set.  Even worse, when you make a shimming adjustment, then re-tighten the lock nut and re-check the alignment, you may discover that the alignment is still incorrect, forcing you to perform the procedure again.

There is another option – permanent alignment spacers (Figure 1).

Figure 1: Permanent alignment spacer mounted to mill spindle

Permanent alignment spacers are just what the name implies – they permanently mount onto each spindle and are made to a specific individual thickness to align all of the spindle shoulders on the machine to a common datum point.  The great thing about these alignment spacers is that you do not need to access the inside of the gearbox or remove any locknuts or bearings.

Note:  Permanent alignment spacers should be at least 0.250” thick and made from hardened steel.  Be sure that the roll space of the mill spindles and the roll tooling can accommodate these spacers. It will be necessary to trim the existing tooling or spacers to accept the permanent alignment spacers.

Figure 2:  Shim packs shown for top & bottom spindles

Incorporating permanent alignment spacers - Process overview
1) Prior to the installation procedure, manufacture one top and one bottom permanent alignment spacer for all spindles.  Use a nominal thickness (0.250” or 0.375” are common).
2) Be sure all bearings are in good condition and are properly set to the recommended torque settings.
3) Be sure to parallel the top and bottom shafts by using the outboard housings and parallel blocks (Figure 3).

Figure 3: Using parallel blocks to prepare shoulders for measurement

4) Using a shoulder alignment tool (Figure 4) or straight edge (Figure 4a), measure the position of each spindle shoulder in relationship to a common (machined) surface of the housing and record these figures on a spreadsheet.

Figure 4:  Use shoulder alignment tool or depth micrometer
Figure 4A: Alternative to a shoulder alignment tool is a straight edge
(Note how rolls are used for a larger surface area to check for gaps - use feeler gages to measure gaps and record on spreadsheet)

5) Using a long precision straight edge, continue to check and record the top-to-bottom shoulder relationships (as well as pass-to-pass) and record each location (Figure 5).

Figure 5:  Use straight edge to check for top/bottom and pass/pass alignment

6) After all the measurements for each spindle shoulder have been recorded, look for the largest number. That spindle will now become the zero point for all the rest of the spindles.  If you are using a 0.250” thick permanent alignment spacer, this will be used here at this position.
7) Take each of the remaining shoulder recordings and subtract that amount from the highest figure.
8) Subtract the results from #7 of each remaining spindle from 0.250” – these figures are what should be ground off the other 0.250” permanent alignment spacers.
9) Stamp or etch the final length and location of each permanent alignment spacer (i.e. 1T - .243”, or 4B - .226”, etc.).
10) Mount all of the permanent alignment spacers in their appropriate positions and double check the alignment by repeating steps #3-5.
11) Retain a record of each of these permanent alignment spacers for future reference.

Using these permanent alignment spacers on specific mills can greatly reduce the amount of maintenance time required to align spindle shoulders.  In addition, replacing or preloading the bearings is more simplified because the only concern is correctly setting preload, not the alignment as well.

If you are experiencing shoulder alignment or bearing preload issues and would like further details, or have other roll formed production problems, Roll-Kraft is your customized solutions provider.  

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