By: Robert A. Sladky
The importance of the driven shafts (tooling spindles) is often neglected during the normal maintenance routine of a tube and pipe mill. One or more bent or excessively worn shafts can create numerous production and quality problems that are difficult to trace and solve. To follow are recommended guidelines to use for inspection, and rework justification for your driven shafts. By instituting this inspection process into your mill maintenance program, you can determine the value to your organization of reworking the driven shafts vs. replacing them. For clarity, the following examples show the spindles removed from the mill housings/towers. Use the same inspection process as described while the shafts are installed on the machine.
O.D. of Shafts
Check the outside diameter (O.D.) of the shaft.
Step 1: Begin by measuring the O.D. of the shaft closest to the tooling shoulder to establish a baseline diameter. Generally the spindles are not worn in this area. (Caution: Sometimes driven shafts are not made to the nominal diameter when originally manufactured and if too far off, (undersize) can cut into your undersize tolerance right from the beginning.)
Step 2: Next, measure the OD of the shaft in the area where the tooling is positioned (normally in the center area of the useable roll space), for this will be the area that most of the wear will occur.
O.D. of Journal End
Check the O.D. of the journal end of the shaft. (If so equipped)
Step 3: Measure the O.D. of the journal end of the shaft for wear, but also look for variations from one end to the other such as taper.
The industry standard for maximum allowable undersize tolerance for the O.D. of a driven shaft is .001" per 1.000" shaft O.D. (Example, a 2.000" O.D. shaft, maximum undersize tolerance, .002", or 1.998" O.D.)
Is the shaft bent?
Note: Before conducting this step (when the shaft is mounted on the mill), make sure that both the inboard and outboard stands have been checked for loose bearings or bearing blocks to avoid the possibility of misidentifying a bent spindle.
Step 4: Measure for concentricity (run out) by placing the spindle between two centers (if not mounted on the mill) and using a dial indicator at the center area of the roll space, rotate the shaft around from one side of the keyway to the other and record the deviation.
The industry standard for the maximum allowable run out of a bent shaft is .001" per 1.000" shaft O.D. (Example, a 2.000" O.D. shaft, maximum run out, .002")
It is easy to understand from the examples above, that if you go to the maximum allowable undersize tolerance, and match that with the maximum allowable run out tolerance, you can potentially have .008" worth of movement (breathing) of the tooling which are mounted on these affected shafts. This will ultimately result in quality problems on the mill line.
Products which require very close dimensional tolerances, and very light gauges will be susceptible to even tighter tolerances outlined in this document.
Driven shafts which have been earmarked for replacement from the previous guidelines can be reworked by grinding down the O.D. of the shafts, applying hard surface chrome, and grinding them back to their original O.D. (Keep in mind that the bearing journal surface damaged by a seized bearing can also be a candidate for hard surface chrome rework.)
The hard surface chrome not only brings back the serviceability back of the driven shafts, but provides long wear and eliminates roll seizure as the chrome surface will not rust or corrode.
In extreme cases of worn or bent shafts, if more than 1% undersized from the original O.D. of the shaft, or .5% in regard to a bent shaft, it will probably not be cost effective to rework the worn/damaged shaft.
Chipped keyways are nearly impossible to repair. Many methods of welding and re-metalizing have been attempted, but subsequently failed, due to the fact that the process affects the hardness and integrity of the shaft.
Shafts with chipped keyways that exceed the following guidelines should not be sent in for rework:
Keyways chipped more than 25% of the total keyway depth in the shaft
Example: 0.500" deep keyway in shaft
25% of 0.500" = 0.125" maximum chip depth allowed
Keyways chipped the length of the shaft that exceed twice the width of the keyway width
Example: 0.750" wide keyway
0.750" x 2 = 1.500" maximum chip length allowed
Threaded Ends or Tapped Holes
Most rework facilities do not repair damaged ends or tapped holes. The ends must be in good condition, so the repair facility can locate on live centers to turn the shafts true during reworking.
If threads are damaged on the shaft ends, chase with a die specifically made for use on existing threads, before sending the shafts in for rework. If threaded ends are broken off, discard the shaft. If bolts are broken off in tapped holes, extract and clean tapped hole with a tap before sending shafts in for rework. In some cases, such as in a breakdown situation, both of these problems can be solved using other alternatives methods. The recommendation to discard the shaft depends solely on the cost difference between repairing and replacing the shaft.
Your efforts in following these guidelines will help you identify the problem driven shafts on your mill and prevent unnecessarily wasted time and money in shipping shafts that should have been "weeded out" before being sent in for repair. Contact Roll-Kraft for additional information regarding this or other machine related maintenance topics.
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