Should Jack Screws be tight or backed off?

byJohn Piotrowski
Posted 1/29/2007

There has always been a debate on whether jack screws should remain tight against the side of the machine case after the machine has been moved into position or to back off the screws, so they do not touch. To my knowledge, there are not any ‘published papers’ on the use and application of jackscrews or jacking bolts but there is some information on this in the 2nd edition of the Shaft Alignment Handbook (ISBN 0-8247-9666-7). Indeed, the primary purpose of jacking bolts is to position rotating machinery in the lateral (sideways) direction during the alignment process.

The answer is yes and no. In some situations you should leave the bolts tight against the machine and in other situations, it may be better to back off the jackscrews.

Here is a situation where opposing jack screws (with a lock nut) remain tight against the machine case. Some machinery (usually very large ones) are held in position sideways with jackscrews pinched against a casting ‘arm’ located on the lower machine case half in the 6 o’clock position. On both sides of the lower casing half of the machine near the centerline of rotation, there are two casting ‘wings’ that rest on vertical supports. The bolts going through each of the wings don’t really pinch the machine case against the vertical supports, rather the underside of the bolt head ‘bottoms out’ against a precisely cut length of pipe or tubing that is inserted in each of the holes in each wing. The purpose of this is to allow for the machine case to expand thermally without distorting the machine case in the process. An example of this in the steel industry can be found on some steam turbine driven compressors used for the air supply to the blast furnaces. Both the steam turbine and compressor are mounted this way. This is also true in the electric generating industry where the main turbine – generator sets have a similar setup.

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Figure 1. Situation where jackscrews should remain tight.

On general process machinery (e.g. motors, pumps, fans, etc.) the general rule of thumb that I use is … once I get the machinery into position, back the jackscrews off 0.010″ (10 mils) and tighten the lock nut. Then, periodically (after 100 hours of operation or so) check to see if there are still 10 mil gaps between the ends of the jackscrews and the machine case. If there is, great, if not, that is an indication that the machine is trying to shift around. This shifting of a machines position can be traced to several things. For equipment with piping or ductwork attached to it, the movement is sometimes due to static piping stresses (i.e. the pipes didnÕt fit right and were pulled or pushed into place) or what I refer to as ‘dynamic’ piping stresses (i.e. the pipes fit correctly when the machine was off-line but expanded or contracted during operation causing a shift to occur). Another major cause of shifting in machinery is due to improper contact between the undersides of a machine foot (or feet) and the points of contact on the baseplate … often referred to as a ‘soft foot’ condition (refer to Chapter 6 in the Shaft Alignment Handbook). It is also possible for the foundation or frame to warp thermally (short term shifts) or for the foundation to settle and change its position over time (long term shifts). One engineer at a Dow Chemical plant in California who has been tracking the alignment of all of their rotating machinery over the last three years indicated to me that over 60% of their machinery shifts its position within a 12 month period of time enough to require re-alignment. Others, stay rock steady and have a repeatable pattern of movement from off-line to running conditions. Hmmmm. LetÕs think about this for a moment however. If a piece of machinery is held down in four corners with bolts, and the machine case gets warm and expands thermally but the baseplate it is sitting on does not change as much in temperature and does not expand thermally, then are we potentially distorting the machine case? Or does the machine case expand and slippage occur between the machine case and the bolts holding it in place? A 1/2″ grade 5 bolt should be tightened to 76 ft-lbs of torque which will produce a clamping force of slightly over 9000 lbs. at each bolting point. Whew! Over 36,000 pounds of force are holding a machine down at four corners to its mating baseplate (assuming it was torque correctly).

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Figure 2. Removable jackscrew plates to allow for lateral positioning of motor. Thank goodness some rotating equipment manufacturers are listening to our requests!

Do you (or your trades people) use a torque wrench to tighten your foot bolts down? Do you know that you are making over 80% contact (no soft foot) between the underside of every foot and its contact point on the baseplate? Do you (or your trades people) keep records of the soft foot problems you found on every machine and how it was corrected? Do you periodically check the alignment of your machinery to see if it is shifting its position? If a jackscrew is positioned by each bolt and the bolt is really holding the machine firmly at that point, what difference would it make if a jackscrew is positioned against the side of the machine there? From a vibration standpoint, having jackscrews touching or not touching should not have any effect on the vibration response assuming the bolts are really what is holding the machine in place, not the jackscrews. Under this premise, should dowel pins ever be used and if so, where and how? (yet another subject I get asked about frequently).

Well Pati, I think the best course of action is to: back off the jackscrews and if a significant shift occurs over time, what is causing it? Finding the source of this shifting can be very difficult to measure for verification and corrective measures to be taken. Best of luck and let us know what you find!

Sincerely … John Piotrowski

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