How to Track Large Rotating Equipment
Mike Sondalini, PWW EAM System Consultant
with permission of BIN95 Business Industrial Network
Posted 12/5/2024
Introduction – How to Track Large Rotating Equipment
Rotary dryers, kilns, mills and reactors turn on tires and trunnions. Each tire is mounted to the rotating shell and revolves on two roller trunnions. If the alignment between the tire and trunnions is lost due to wear, poor repair, installation error or impact, the equipment will need to be tracked.
The shell is rotated mechanically. Correct axial and linear positioning of the tires and trunnions aligns the rotating shell. Guide rollers are fitted to the front and back of a tire and trunnion set to limit axial movement. Generally, the tires and trunnions are made of steel but in lightweight equipment with low rolling surface temperatures the trunnions can be lined with hard wearing plastic. In those situations where the product is continuously fed through the equipment the entire shell is inclined a few degrees. With each rotation the product is lifted and moved along a distance.
AXIAL ALIGNMENT
Installation and maintenance teams will flounder when tracking the tires without proper references to align the equipment. Proper tracking of rotary equipment requires access to a datum from which the axial alignment of the tires can be set. The datum is a
series of points whose relative vertical and horizontal positions are accurately known. The sketches below show how the datum is used to set the vertical and horizontal position of the tires.
Unless the equipment manufacturer or installer provided the datums, you need to be creative in establishing the necessary reference points. One method that proved suitable on a rotary reactor was to use laser surveying to establish both the vertical heights of the trunnion pedestals under each tire and to position a datum line offset from the tire axis to provide horizontal reference points. This allowed accurate measurement of the horizontal and vertical position of each tire axis to within 1mm.
The following sketch shows how the hold down bolts were used as a horizontal reference on the assumption that the pedestals were cast identical and had been properly aligned when the equipment was originally installed. The numbers in the circles represent the vertical datum and show that all but one end of one pedestal were at the same level. The “marks on the concrete floor” distance represent the offset from the hold down bolts to give the horizontal alignment. Provided the drive alignment can be adjusted it is simpler to line up the tires and trunnions to the datum and then align the drive to the shell.
ROLLING FACE ALIGNMENT (Training the kiln)
A further consideration in tracking large rotating equipment is the alignment of the trunnions to the tire.
The tire and trunnion must always be aligned to each other. Where the shell is inclined the trunnions must also be inclined to the same angle. If the rollers are set skew to the tire axis forces are created which drive the shell along its axis. This can advantageous and necessary if the shell requires ‘floating’ between the axial guide rollers but leads to unnecessary wear otherwise. Misalignment between tire and rollers can cause excessive wear of the rolling faces because the previous line contact between the two surfaces becomes a highly stressed point contact. Offsetting one trunnion pair from the other will cause twisting of the shell. The sketches below highlight the problems caused by rolling face misalignment. Correct tracking of rotating dryers, kilns and reactors is achieved by first aligning the tire and trunnion sets axis true to each other.
It is easier if all trunnions are the same diameter otherwise compensating calculations must be made for each different sized trunnion. The shell and tires are lowered or raised by jacking the trunnions respectively out or in. Axial thrusting is then removed by giving the trunnions a miniscule amount of offset relative to the tire axis. Axial ‘float’ adjustments are done by jacking alternate sides of each trunnion pair in or out depending on which direction the float is required. A ‘rough-in’ adjustment can be done while the shell is cold, but the final setting must be done at operating temperature while under load.
The drawing below shows one of the two possible orientations of the trunnions to create axial movement of the shell. One way to understand what the trunnion offsets does is to think of the shell with an imaginary helical ‘thread’ on its outside. By skewing all four trunnions minutely they act like a nut running along the ‘thread’. But since the trunnions are stationary it is the shell that gets the thrusting action from the trunnions and moves along the ‘thread’.
To select the trunnion bearing housing to be jacked stand behind the trunnion concerned with the fingers of both hands pointing in the opposite direction to the rotation of the shell. The hand with the thumb pointing in the direction the shell must be steered is the same hand as the bearing housing that needs to be jacked minutely inward to the shell. Set-up dial indicators at the shell end of each bearing housing to be jacked. Jack both bearing housings on each trunnion pair at the same time by equal amounts. Move the bearing housings 0.025-mm (0.001”) at a time. Continue the equal adjustment till the tire moves away from the axial thrust guide roller. As little as 0.1 mm (0.004”) can suffice to steer the shell.
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