Aligning and Bedding Down Plant such as Machine Tools

Kitmondo 02 May 2014

This article first appeared in Practical Engineering 1940 Vol1 No19. The information contained herein is therefore correct as of 1940. The publication attempts to provide historical insight into methods prevalent in the Manufacturing, Process and Engineering sector during this period.

When arranging or re-arranging the plant in a machine shop the question of foundation and alignment is one of importance. Not all shop floors are dead level, but all shafting is obviously erected level and this is done with a long spirit level laid along the shaft, whilst brackets, hangers, pedestals, bearers, etc., are adjusted or packed up to make the shafts level.

The bedding-down of the machine and its alignment is therefore always carried out with the line shaft as the basis, and obviously the driven shaft of the machine must be level, i.e., parallel with the line shaft.

Lining Up Countershafts

Most machines arc driven from counter-shafts having fast and loose pulleys geared by belt to the line shaft, with striking gear so that the belt can be shifted along the wide driving pulley on the shop line shaft, and engage either the fast or loose pulley on the countershaft. The countershaft must, therefore, be in the same plane as the line shaft or the belt will run off, i.e., the line shaft and the countershaft must be parallel and in the same plane. This plane need not be horizontal, the line shaft may be above or below the countershaft. An instance in which the countershaft may be below the line shaft is the vertical drilling machine, where the countershaft is often built into the base of the machine.

The countershaft carries the pulley, by means of which it is driven, the loose pulley rotating idly on the same shaft also the cone pulley by means of which different gear ratios may be obtained between the countershaft speed and the speed of the machine. Usually (as in the case of lathes) there are three steps; in some cases there are four.

Direction of Drive

In arranging the drive from the line shaft to the countershaft it is of great advantage if the machine to be driven can be placed so that the pull of the belt is on the underside and not on the top. This stretches the belt between the two pulleys in taking the drive. The bottom bight sags down and leaves the pulleys—sags away from them. When the bottom run of the belt takes the pull, the sag is on the top and the belt hangs on to the pulleys so that the part of the pulleys in contact with the belt is increased and the drive is more efficient with less strain on the belt. When the top run of the belt takes the pull the bottom run sags away from the pulleys and the area of contact is considerably reduced, giving a less efficient drive.

It is not always convenient to arrange this as regards belts from line shafts to countershafts, since economy of shafting and power conservation, as well as convenience and space saving, can best be obtained by aligning the line shaft between two rows of machine tools —a single line shaft for every two rows of machines. The line shaft should be levelled by the spirit level, and this becomes the datum line for all the machines driven from it.

Plummer Blocks and Bearings

Line shafts are usually hung by hangers and plummer blocks—bearing blocks—from roof joists. Where the shop is wide and the roof supported by pillars, these pillars can carry brackets. In the building of a large shop the pillars of cast iron (the equivalent rolled steel or riveted stanchions) are provided with vertical surfaces for the bolting-up of the bearing brackets, which have slots for the studs or bolts to allow for vertical adjustment in lining-up the shafts to horizontal.

Hangers or brackets should be near enough to each other to prevent shaft spring. The spacing between hangers may be 10ft. in most cases. Where heavy load machines are driven it is advisable, if possible, that they should be located so that their driving pulleys on the line shafts are near a bracket or a hanger. The lighter drive loads can then be taken from the centre of the line shaft span.

It is not good practice to have the driving shaft or the countershaft immediately above the driven shaft or pulley. The weight of the belt tends to cause it to leave the surface of the bottom pulley. It may thus only be in frictional driving contact with the pulley for perhaps one quarter of its diameter. This necessitates a very wide belt, or undue tension in the belt between the pulleys on both the driving and slack sides. The ideal angle is 45 deg., but this is not always available and causes obstruction, especially where the driving pulley of the tool is on the base (as in most drilling machines).

Use of the Plumb Line

The alignment of the countershaft pulley and the machine driving pulley must be correct in two planes. Their shafts must be parallel in both planes. The simple method is to drop a plumb line from across the edges of the countershaft pulley and line it with the edges of the driven pulley on the machine. The axis of a machine pulley must be level, and since it is always true with the machine table, a spirit level on the table, the bed of a lathe, the table of a drilling machine, the top of the knee of a shaper, or the work table of a milling machine will give the necessary surface. The level should be used in two directions at right-angles to each other.

The plumb line along the edges of the pulleys may be true, but the pulleys may still be out of alignment if the machine is turned a little sideways. To test for this, drop the plumb bob on the floor from two positions of the countershaft—each end is convenient. Join these points by a chalk mark on the floor, then align the drive shaft on the machine with this chalk mark by measurement. In the case of the lathe, if a plumb line is dropped from each end of one edge of the bed and marked on the floor, and these points joined by a line, a line is given which should be parallel with the line of the countershaft.

In the case of other machines, lines can be dropped from the ends of any shaft in the machine which is parallel to the driving pulley shaft of the machine —the headstock mandrel of a milling machine, for example. In thus locating a machine for alignment it is useless to rely on the feet or supports for getting a line. They may be well out of square or alignment with the essential running shafts and spindles of the machines.

Having got the position by placing the machine so that it is in proper alignment with the line shaft or countershaft, and packed up if necessary to get the bed or table level (and running it if the power is on to see that the belt does not run off), mark round the pedestal or feet with a piece of chalk and mark with chalk the centres of the holding-down bolt holes. Then move the machine back a corner at a time so that these marks are unobstructed.

The Holding-Down Bolts

The cement floor is now cut out by a chisel and hammer in a roughly square hole that will come partly under the foot or pedestal at the bolt hole. Cut from the front so that the chalk marks remain on at least two sides of each hole so that the machine can be replaced to the chalk marks with part of its feet standing on the cement floor and a hole directly below the bolt hole and extending out sideways, or back or front, to give a passage at the side of the foot or pedestal for pouring in the cement. The hole should be at least 4in. deep, and for heavy machines 6in. deep. It should be undercut, and the walls should incline outwards towards the bottom of the hole so as to form a key for the cement. The bolt should be 1in. for a machine such as a 5in. lathe; bigger bolts for bigger machines and deeper holes.

Thread a roughly square plate on the bolt up to the head, and insert it down into the hole, bringing the screwed end of the bolt up through the hole in the pedestal or leg foot (according to the style of the machine). Then oil the bolt thread and slip on a washer and a nut screwed down level with the end of the threaded end of the bolt.

The holding-down bolts will now be hanging in their holes with their plates on. Now check that the machine is in position by the chalk marks. Put a level on the lathe bed or other machine table and pack up if necessary under the pedestal or feet with pieces of 1/8in. or 1/4in. iron plate. Now break up stone or brick or old broken cement and lay this loosely in the hole, around the bolt, above and below the plate and up to within 1/2in. of the floor surface. Mix Portland cement with fine sharp sand (not sea sand) in equal proportions and to the consistency of cream, so that it will pour round the bolt and around the pieces of broken stone, brick, or cement, and fill the hole up level. If the packing has raised the machine slightly above the floor, build the cement up around the leg.

Let everything stand for 48 hours if possible, occasionally, after it has hardened slightly, swilling cold clean water upon it. This causes it to set more slowly and more firmly. The machine is now fixed, and all that is necessary is to give each of the nuts a turn until they are spanner tight. Do not tighten the nuts until the cement has set hard, or the whole work of cementing will be wasted and all will have to come out and new cement will be required.