Readings that will not repeat
If one set of readings says move left and the next says move right, something is moving that you are not controlling. It is nearly always something loose, not the measuring tool.
Three places to look
Sensor and bracket
- Brackets tight on the shaft or hub, rods tight in the brackets, sensors tight on the rods.
- Magnetic bases making full contact.
- Is the hub tight on the shaft?
- Is anything rubbing, or did someone bump the set-up?
- Sleeve bearings: has the shaft settled to rest before you read?
- Clean sensor lenses, if you use laser heads. Grease and grime stops the beam being seen.
Base and feet
- Do the feet move on the base when a bolt is tightened?
- Does the base itself move?
- Debris under feet or shims? A loose, cupped or broken washer?
- Too many shims? Can the stack be reduced?
- Hidden shims, a broken foot, or a wedge under a foot.
- Jacking bolts left tight against the feet.
Torque and play
- Worn or loose coupling element, or hub fit.
- Are you turning the shaft the same way each time?
- Gear backlash in the train.
- A shaft that rolls on its own when the sensors are at the side.
- Worn motor or pump bearings.
- Pipe, conduit or ductwork pulling on the machine.
Hardware that quietly moves the machine
- Cupped washers. A thin washer under an oversize hole bowls as the bolt tightens, then centres the bolt in the hole and drags the foot with it. Each tighten and loosen cycle moves the foot again, and the alignment looks as if it has soft foot. Replace with thicker washers or a double stack and check the bolt hole.
- Broken or undersized washers. A cracked washer means a loose bolt. Match the washer size to the bolt and do not choose washers purely on price.
- Jacking bolts left tight. After the alignment, back them off. Tight jacking bolts bind the feet and can add soft foot or distort a motor frame. One field example saw vibration fall by about a quarter in the horizontal and axial directions once they were backed off. A jam nut with a thin shim gap keeps the machine from drifting without binding it.
- New bolts. Bolts stretched by many tightenings can behave differently. If the job is stubborn, replace them.
Shaft movement while you adjust
Holding a motor shaft by hand to steady it does not work: even a heavy motor shaft creeps. Large motors can also roll freely one way and resist the other because of residual magnetism. Rest the sensors on a simple steady rest or counter-weight the sensor, so nothing rotates while you move the machine. On a barge, ship or any floating or swaying base, the whole machine rocks slowly, and an inclinometer will see it as a false error.
Vibration from the machine next door
Run a repeatability test before you start: take several sets of readings without moving anything. If the coupling values vary by less than half your tolerance, you can align despite the vibration. If they vary more:
- Check pipe hangers, isolators and base bolting for how the vibration gets in.
- Increase the measurement time or turn on the filter on your tool.
- Shut the neighbour down for the few minutes of measurement if it is possible. A machine that shakes the building so much that you cannot align next to it probably needs attention itself.
Is the machine alignable at all?
A simple lift test tells you whether a bearing has play. With the sensors at the top, lift and release each shaft lightly by hand and watch the displayed value. A healthy shaft moves a few tenths of a mil (about 0.01 mm). One motor moved five mils (0.13 mm), and its centreline changed from one reading to the next. No procedure could align it to a tight tolerance until the bearings were replaced. In another case one pump foot had broken off and was resting on a piece of wood that acid had eaten away.
Questions
Why do my alignment readings keep changing?
Something is moving: the sensor brackets, the machine feet or washers, the coupling, or the shaft itself. Soft foot and loose hardware are the usual suspects.
How do I test whether vibration will ruin my alignment?
Take several sets of readings without changing anything. If the values vary by less than half your tolerance, it is safe to proceed.
How do I check a motor for bearing play before alignment?
Rotate the sensors to the vertical, then lift and release the shaft gently and watch the displayed value. A shift of a few tenths of a mil is normal. Several mils means worn bearings.
Should jacking bolts stay tight after alignment?
No. Back them off, or use a jam nut set a few mils from the foot. Tight jacking bolts can add soft foot and distort the machine.
General good practice for horizontal machines driven through a coupling. The machine maker's manual, the coupling data and your site standard come first. Lock out and isolate the machine before any alignment work.