Chuck Yung
EASA Technical Support Specialist
When vibration problems occur, the magnitude and direction of the vibration can give a good indication of where to look for the cause. When vibration is higher in the vertical plane, one of the first things we should examine is the base/foundation of the motor. If the high vertical readings are compounded by indications of an eccentric airgap, such as high axial vibration and a predominant twice-line-frequency vibration, a “soft foot” or twisted frame is often to blame.
Construction basics
It is common practice for the alignment technician to use prefabricated shims under the feet, sized to accept the hold-down bolt. The person performing the alignment may not realize that a motor frame is not as solid as it appears. The fact that the foot itself might be over an inch (25 mm) thick, and the frame is cast iron or steel, causes the person to assume that it cannot distort. Nothing could be further from the truth. Because of that assumption, shims are often not placed to the greatest benefit. By understanding some construction basics, we can better place the shims to obtain the lowest vibration readings.
Figure 1 shows a typical fabricated frame construction. The holes through the feet, sized for the hold-down bolts, are indicated by arrows. Depending on motor size, there are at least 4 bolts, and sometimes more. The feet may be a standard pad at each corner of the motor, or may run the full length of the frame in which case there are usually 4 or more bolts along each side of the motor.
Support bulkheads
The bolts cannot be too close to the support bulkheads of the motor to facilitate access to tighten them. The bolts pass through a solid-looking foot, which is securely joined to the support bulkheads during the fabrication process. But supporting those bulkheads is critical to minimize vibration and frame distortion. If you can change vibration levels by loosening/tightening base bolts, you prove that the motor is not properly shimmed.
Examine a dismantled motor with a fabricated frame. The bulkheads supporting the stator inside the frame, and the bulkheads that help channel the internal airflow in WP (weatherproof) enclosure machines, do more than simply hold the stator inside the frame. They are also stiffeners for the entire frame and help to dampen vibration. When one or more of those bulkheads is unsupported, vibration levels will be higher than if they are properly supported.
In Figure 2, the critical areas for shimming are indicated by arrows. When a corner requires x amount of shim, that shim should extend to the area directly beneath the bulkheads near the bolts. If the base and supporting foundation are not parallel,the shim thickness required beneath the bulk h ead may differ from that required at the bolt.
Minimizing vibration
After the bolts have been tight-ened, the shims beneath the bulkheads should not be loose. If the soft-foot check procedure of loosening the bolts indicates a soft foot, there may be too much shim beneath the bulkhead. When the feet are correctly supported with the entire foot shimmed, includ- ing the area directly beneath each bulkhead (Figure 3), vibration will be minimized. Note that these same basic principles also apply to other rotating equipment: generators, blowers, screw compressors, etc.
Handling of a fabricated-frame motor during installation, removal and repair, should always utilize every lifting point provided by the manufacturer. If a motor with a welded steel frame is designed to be lifted at all four corners, picking it up at only two diagonally opposite corners is likely to twist the frame. Assembly and
By making sure the alignment technician understands these simple principles, you can help your customers reduce vibration levels.
irregular airgap, and a twisted frame causes future soft foot problems, both in the service center and on the job site.
Possibility of frame distortion
Prudent engineering often uses a safety factor of 5 (depending on many factors, such as the consequence of failure), so while half the lifting points might safely lift the motor without failing, frame distortion is still a very real possibility. Caution: That does not mean that a motor can be lifted using only half the lifting points.
The bottom line is that minimizing vibration often depends as much on where we place the shims as it does on our using the correct amount of shims. By making sure the alignment technician understands these simple principles, you can help your customers reduce vibration levels.
ANSI/EASA AR100
More information on this topic can be found in ANSI/EASA AR100
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