Motor base support and alignment

A basic understanding of frame construction can help minimize motor vibration.

10/22/2013


When motor 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 to examine is the base/foundation of the motor. If the high vertical readings are compounded by indications of an eccentric air gap, such as high axial vibration and a predominant twice-line-frequency vibration, a “soft foot” or twisted frame is often to blame.

Construction basics

Figure 1: The base bolts fasten the motor to the soleplate. Courtesy: EASATo correct “soft foot” conditions, alignment technicians use prefabricated shims (sized to accept the hold-down bolts) under motor feet. What they may not realize is that a cast iron or steel motor frame isn’t as solid as it appears. Because the feet are typically at least 1 in. (25 mm) thick, technicians mistakenly assume distortion isn’t possible. As a result, they do not place shims to the greatest benefit. A review of motor frame construction basics can be helpful in determining where to put shims to obtain the best support, and therefore the lowest vibration readings.

Figure 1 shows the typical fabricated-frame construction; the holes through the motor feet are sized for the hold-down bolts. Depending on motor size, there will be at least four bolts (one at each corner of the motor), and sometimes more. Each foot may be the customary weldment at one corner of the motor, or it may run the full length of the frame, in which case there are usually four or more bolts along each side of the motor.

Frame bulkheads

An exploded view of any fabricated-frame motor would clearly show that the bulkheads do much more than hold the stator in the frame or channel the internal airflow, as in the case of a weather-proof (WP) enclosure machine. They also stiffen the entire frame, helping dampen vibration. Vibration levels will therefore increase if one or more of the bulkheads isn’t properly supported by the motor base. Visualize a cantilevered or overhung mechanical structure and the resultant resonance.

Figure 2: The bulkheads (arrows) support the stator and end brackets and stiffen the entire motor. Shims should extend beneath the bulkheads as well as the mounting bolts. Courtesy: EASAAlthough most motors have solid-looking feet that are securely attached to the bulkheads, the mounting bolts must be far enough from the bulkheads to provide sufficient access for the mechanic to tighten them with a wrench. Therefore the bulkheads (as well as the motor feet) may require shimming to achieve the rock-solid support that is critical for minimizing frame distortion and vibration. If loosening or tightening base bolts changes the vibration levels, the motor is not shimmed properly.

Figure 2 illustrates the critical areas for shimming. When a corner requires X amount of shim, that shim should also 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 bulkhead may differ from that required at the bolt.

Minimizing vibration

The shims beneath the bulkheads should not be loose after the mounting bolts have been tightened. If the “soft-foot check” procedure (see sidebar) of loosening the bolts indicates a soft foot, the shim pack beneath the bulkhead may be too thick. Vibration will be minimized once all feet are correctly supported, including the area directly beneath each bulkhead (see Figure 3). Note that these same basic principles also apply to other rotating equipment–e.g., generators, blowers, screw compressors, and so forth.

Avoiding frame distortion

Figure 3: Lack of support beneath the bulkhead (left) will result in higher vibration than when the bulkhead is shimmed correctly (right). Courtesy: EASAWhen installing, removing or repairing a fabricated-frame motor, always use every lifting point provided by the manufacturer. Prudent engineering often uses a design safety factor of 5 (depending on many considerations, including the consequence of failure), but it is a mistake to infer that it is “safe” to lift a motor with only half of its lifting points.

If a welded steel frame motor is designed to be lifted at all four corners, picking it up at only two diagonally opposite corners is likely to twist the frame. The resulting irregular air gap will probably show up as vibration at twice line frequency, with higher than expected peaks at 2x RPM in the vertical and axial planes. A twisted frame will also cause future soft-foot problems.

Checking the soleplate

If a machine still vibrates after applying these tips, inspect the soleplate. The same torque that rotates the shaft in one direction is also trying to rotate the frame in the opposite direction. (“For every action, there is an equal but opposite reaction.”) So if the shaft is turning clockwise at the drive end, examine the right-hand corner of the foundation on the drive end of the motor.

Summing it up

The bottom line is that minimizing vibration often depends as much on shim placement as it does on using the correct amount of shims. Using proper lifting techniques can also help prevent frame distortion that could worsen soft-foot and vibration problems. Alignment technicians who understand and apply these basic principles can be more effective in reducing the vibration levels of both motors and driven equipment.

Chuck Yung is a senior technical support specialist at the Electrical Apparatus Service Association (EASA), St. Louis, MO. EASA is an international trade association of more than 1,900 firms in 62 countries that sell and service electrical, electronic and mechanical apparatus.



Top Plant
The Top Plant program honors outstanding manufacturing facilities in North America.
Product of the Year
The Product of the Year program recognizes products newly released in the manufacturing industries.
System Integrator of the Year
Each year, a panel of Control Engineering and Plant Engineering editors and industry expert judges select the System Integrator of the Year Award winners in three categories.
September 2018
2018 Engineering Leaders under 40, Women in Engineering, Six ways to reduce waste in manufacturing, and Four robot implementation challenges.
July/Aug
GAMS preview, 2018 Mid-Year Report, EAM and Safety
June 2018
2018 Lubrication Guide, Motor and maintenance management, Control system migration
August 2018
SCADA standardization, capital expenditures, data-driven drilling and execution
June 2018
Machine learning, produced water benefits, programming cavity pumps
April 2018
ROVs, rigs, and the real time; wellsite valve manifolds; AI on a chip; analytics use for pipelines
Spring 2018
Burners for heat-treating furnaces, CHP, dryers, gas humidification, and more
August 2018
Choosing an automation controller, Lean manufacturing
September 2018
Effective process analytics; Four reasons why LTE networks are not IIoT ready

Annual Salary Survey

After two years of economic concerns, manufacturing leaders once again have homed in on the single biggest issue facing their operations:

It's the workers—or more specifically, the lack of workers.

The 2017 Plant Engineering Salary Survey looks at not just what plant managers make, but what they think. As they look across their plants today, plant managers say they don’t have the operational depth to take on the new technologies and new challenges of global manufacturing.

Read more: 2017 Salary Survey

The Maintenance and Reliability Coach's blog
Maintenance and reliability tips and best practices from the maintenance and reliability coaches at Allied Reliability Group.
One Voice for Manufacturing
The One Voice for Manufacturing blog reports on federal public policy issues impacting the manufacturing sector. One Voice is a joint effort by the National Tooling and Machining...
The Maintenance and Reliability Professionals Blog
The Society for Maintenance and Reliability Professionals an organization devoted...
Machine Safety
Join this ongoing discussion of machine guarding topics, including solutions assessments, regulatory compliance, gap analysis...
Research Analyst Blog
IMS Research, recently acquired by IHS Inc., is a leading independent supplier of market research and consultancy to the global electronics industry.
Marshall on Maintenance
Maintenance is not optional in manufacturing. It’s a profit center, driving productivity and uptime while reducing overall repair costs.
Lachance on CMMS
The Lachance on CMMS blog is about current maintenance topics. Blogger Paul Lachance is president and chief technology officer for Smartware Group.
Material Handling
This digital report explains how everything from conveyors and robots to automatic picking systems and digital orders have evolved to keep pace with the speed of change in the supply chain.
Electrical Safety Update
This digital report explains how plant engineers need to take greater care when it comes to electrical safety incidents on the plant floor.
IIoT: Machines, Equipment, & Asset Management
Articles in this digital report highlight technologies that enable Industrial Internet of Things, IIoT-related products and strategies.
Randy Steele
Maintenance Manager; California Oils Corp.
Matthew J. Woo, PE, RCDD, LEED AP BD+C
Associate, Electrical Engineering; Wood Harbinger
Randy Oliver
Control Systems Engineer; Robert Bosch Corp.
Data Centers: Impacts of Climate and Cooling Technology
This course focuses on climate analysis, appropriateness of cooling system selection, and combining cooling systems.
Safety First: Arc Flash 101
This course will help identify and reveal electrical hazards and identify the solutions to implementing and maintaining a safe work environment.
Critical Power: Hospital Electrical Systems
This course explains how maintaining power and communication systems through emergency power-generation systems is critical.
Design of Safe and Reliable Hydraulic Systems for Subsea Applications
This eGuide explains how the operation of hydraulic systems for subsea applications requires the user to consider additional aspects because of the unique conditions that apply to the setting
click me