How long can a machine tool spindle be stored?
Some of our customers have spare spindles and want to keep them in storage until they are needed. The question then becomes “When does the warranty period start for a spindle in storage” and “How long can a spindle stay in storage”.
Those are good questions. Unfortunately the answer is an ambiguous “it depends”. Here we have some of the considerations and tips for long term spindle storage.
Every spindle that is repaired by HST is prepared for transit and limited storage.
1) Bearing have been lubricated as per manufacturer’s and application recommendations.
2) Any residual coolant is blown out with clean dry air,
3) Spindle is given a final visual examination and wipe down
4) Bare steel areas are sprayed with an industrial rust inhibitor
5) The spindle is wrapped in Cortec Vapor Phase Corrosion Inhibitor (Mil-Spec B220119/020) or equivalent.
6) The corrosion inhibiting wrap is wrapped with a water resistant plastic shrink wrap.
7) The spindle is surrounded by vibration absorbing closed cell foam.
Even well protected spindles can be stored for a only limited time without requiring some sort of additional preparation prior to normal operation. Several things happen that work against the spindle while in storage.
Over time lubrication begins to run off of the metal parts. This is particularly true with oil lubricated spindles. However even grease lubrication will begin to creep off of the bearings over an extended time subsequently exposing bare metal. And grease lubrication will bleed leaving the base substance with no lubricating oil. The higher the storage temperature, the faster the lubricant protection degrades.
The spindle “breaths” as ambient temperature changes. And even when it is stored in clean dry air, bare metal parts will begin to oxidize (rust) when exposed. This condition is exacerbated in warm damp conditions. Corrosion on the highly polished rolling element surfaces will result in premature bearing failure.
Lubricants also have a “shelf-life”. That is over time they start to evaporate, oxidize, separate (grease) or just plain beak down. Environmental conditions will affect the usable life of the lubricant. Unfortunately there is very little published data concerning shelf-life for lubricant used by bearing manufacturers. However, most of the data available indicates that the lubricant shelf-life exceeds two years if stored in an appropriate environment.
Brinelling and False Brinelling
Besides oxidation and corrosion, stored spindles are also susceptible to Brinelling and False Brinelling. In a properly assembled spindle the bearings are preloaded. That is they already are in elastic deformation. Brinelling (true Brinelling) can occur as a result any impact to the housing or shaft. This is an immediate result of mishandling the spindle. So always treat the spindle carefully avoiding and jarring or impacts.
False Brinelling occurs over a period of some time. It is a result of vibrations. When a spindle is stored near vibrating machinery such as an air compressor, the vibrations can travel through the concrete floor and reach the spindle. After some time the preloaded rolling elements will begin to Brinell the races.
False Brinelling is somewhat insidious inasmuch as it might not be evident when the spindle is initially started up. However, it will result in premature bearing failure. The bearing failure may precipitate additional damage that disguises the original False Brinelling.
In general we believe that a properly cushioned spindle can be stored for up to six months in a suitable environment without performance degradation. If the spindle is permanently grease lubricated we recommend rotating the shaft several times every 90 days and leaving it oriented 180o from where it was started. This will redistribute any grease that has settled down.
If the spindle is oil lubricated we recommend rotating the shaft a few times every 30 days. This will help to redistribute any oil that has settled down. If the spindle will be stored for more than 6 months, hook up a suitable air/oiler and letting it run for a ½ hour while rotating the shaft. This will replenish any oil that has completely run off the bearings. When starting up the spindle let the air oiler run for 10 minutes while turning the shaft very slowly. Then increase speed incrementally.
Because we do not have control of the storage environment and because False Brinelling is very difficult to detect after a catastrophic bearing failure we do not normally extend our warranty period even though the spindle may have sat in storage for most of the time.
That having been said, we stand 100% behind our work. So if in our forensic examination we can conclusively determine that the failure was a result of our workmanship (e.g., missing or sheared O-ring) we will honor the warranty.
HST provides a storage service where we will store your spindle in a climate controlled clean area away from vibrating machinery. We will rotate the shaft periodically and reload oil lubrication spindles as needed.
When your spindle is needed we will run-up your spindle to full speed and verify that it passes all tests before shipping. Your warranty period will start after we ship the spindle.
1) Spindles have a limited shelf life before certain maintenance procedures are required in order to maintain proper performance
2) Spindles must be handled with care to avoid Brinelled bearings.
3) Store spindles in a climate controlled environment
4) Rotate stored spindles periodically to redistribute lubricant that may have settled
5) Store spindles away from any vibrating machinery
6) Consider using HST Spindle Storage Services for long term storage of your spindle.