Basic Troubleshooting of Common Rotating Equipment
Technology has brought us many fantastic monitoring and troubleshooting tools such as vibration measurement, ultrasonic testing, power quality measurement, oil sampling, torque monitoring using strain gage technology, thermal imaging cameras, laser alignment equipment et. al. Certainly there will be situations where using one or more of these approaches will be beneficial when trying to identify problems. But even then, once the root cause of a problem is identified, problem solving only begins by using a best-practice strategy when it comes to installation and maintenance. As example, assume excessive vibration, as found through a detailed vibration analysis, is found to be the cause of a bearing failure on a pump application. The study further reveals the motor and pump shafts are out of alignment and the resulting vibration is causing premature bearing failure. It’s great that the cause of the problem was identified. However, if best practices with shaft alignment were conducted at equipment installation, the use of sophisticated equipment would not have been necessary and equipment downtime could have been avoided. Sometimes, it just makes sense to take a back-to-basics approach to installation and maintenance. It’s amazing how many drive problems you can solve simply by using your sense of sight, hearing, and touch, combined with an understanding of power transmission products. In fact, oftentimes, more is learned about the performance of the equipment after a failure has occurred than during operation. Some of the more common rotating equipment failure modes, the causes of the failure, and how to avoid are presented in the following sections.
Although simple in design and operation, V-belt drives are often installed and/or maintained improperly. Three primary problems that customers encounter with these drives are:
- Improper initial and after ‘run-in’ V-belt tensioning.
- Improper installation of tapered bushings.
- Sheave misalignment.
Maintaining proper tension is the most important rule of V-belt performance. AND, it is not only important to establish the proper tension during initial installation, it is also important to check and re-qualify tension at least once during the first 24 hours of operation. When a V-belt drive is under-tensioned, the V-belt slips or moves independently of the sheave grooves during operation and this action causes a loss in efficiency and premature wear on the V-belts and sheaves.
Under-tensioned V-belts are easily detected and these are the symptoms:
- A ‘squealing’ sound during start-up of drive.
- A ‘chirping’ sound during operation.
- The sides of the V-belts will display a ‘shiny’, or glazed, appearance.
- There will likely be black-colored dust or residue, as a result of the V-belt wear, in the bottom of the V-belt guard or shroud.
- The V-belt will become hardened and perhaps display cracking on the underside.
Cracked underside from heat aging
‘Shiny’ sidewall from slippage
Improper Installation of Tapered Bushings
V-Belt sheaves, synchronous pulleys, and other tapered bore components often fracture and a user may report that “the hub cracked during installation”. This failure occurs either because the bushing cap screws were over-tightened or a lubricant was applied to the tapered bore of the sheave or the barrel of the bushing. The solution to prevent hub fractures on tapered-bore sheaves and hubs merely requires NOT applying lubricants of any kind on the tapered surfaces or cap-screws and tightening the bushing cap screws to the correct torque value as guided by the product instructions. A customer may think that lubrication will make the disassembly easier at a later date. However, lubricants reduce the friction between the mating parts allowing the bushing to wedge deeper than intended into the bore and the outward ‘push’ results in hub breakage.
Sheave broken through cap screw hole
Lubricant in tapered bore
Besides presenting performance problems, misaligned sheaves can cause rapid wear on the V-belt sidewalls and can even prematurely wear the grooves in sheaves. Some things to look for when checking for misalignment include:
- The V-belts (on multiple V-belt drives) appear to be mismatched
- One side of the V-Belt may be more worn than the other
- One side of a sheave groove may appear more worn or shinier than the other side
- A ‘chirping’ sound may be heard during operation because one or more V-belts may be ‘loafing’
An easy method to check or qualify sheave alignment is to place a straight edge along the sides of the sheaves. The goal here is for the straight edge to make, or come very close to achieving, four-point contact with the sheaves. While this is easy to do, an often overlooked technique is only checking for this feature above the shafts. It is very important to check this criteria for four-point contact both above the shafts AND below the shafts.
If a straight edge item is not available, a string may be drawn taut to check for alignment as well and this will be ideally used for very long center-distance drives where a straight edge may not be available. Remember that when using the string method, it is important to also pull the string on the reverse sides of the sheaves with the goal of achieving eight-points contact. Make sheave adjustments as appropriate to achieve the best alignment possible.
Typical performance problems with gear reducer drives requiring troubleshooting include:
- Excessive noise and/or vibration
- Oil leakage
- Loose shaft
- Premature bearing failure
A gear reducer may be reported as running ‘hot’. Please understand that just because the housing (enclosure) of a gear reducer feels very warm when touched by hand does not necessarily mean the unit is operating in an overheated condition. It’s best to take temperature readings at several locations on the gear reducer. This is easiest accomplished using an inexpensive hand-held infrared thermometer. Normally the hottest area on the gear reducer will be near the input shaft since it rotates at the fastest speed. The lubrication and rubber lip oil seals of gear reducers, both of which are challenged at higher temperatures, are typically designed to withstand temperatures of 200+ °F. A good rule of thumb is the external housing of a gear reducer should not exceed 100°F above the ambient temperature. So since a ‘too hot to touch’ sensation for the average person can be around 135-140°F, it’s easy to understand why this can be a false indicator of performance.
|Probable Cause of Overheating||What To Do|
|The load demands exceed the mechanical and/or thermal capacity of the gear reducer.||Decrease load if possible. Otherwise, replace unit with one of sufficient mechanical and thermal capacity. Add a fan if thermal limited only. Verify overload by taking current measurements on the motor while in normal operation and compare the reading to the FLA value noted on the motor nameplate.|
|Insufficient oil level.||Raise level to mfrs. recommendations.|
|Too much oil.||Drop level to mfrs. recommendations.|
|Vent plug not installed or clogged.||Verify vent plug is installed correctly and clean as appropriate.|
|Wrong viscosity oil.||Drain oil and replace with correct viscosity.|
Excessive Noise and/or Vibration
Abnormal noise being generated from a gear reducer may present a variety of sounds. The more common problems present themselves with a ‘knocking’ sound that is normally caused because of broken or damaged gear teeth. A ‘clunking’ sound is often associated with bearing wear or damage.
|Probable Cause of Excessive Noise||What To Do|
|Improper installation.||Review installation manuals.|
|Worn gearing and/or bearings.||Conduct oil sampling procedure. Seek service as needed.|
|Improper connection with other machinery.||Verify that shaft alignment satisfies the requirements for the equipment.|
Oil seals in gear reducers are wear items much like the brakes/tires on an automobile. However, this wear can be accelerated where conditions of very warm ambient temperatures are present, a gear reducer is overloaded leading to overheating, and where damage is likely due to interference from debris. When oil seals get hot, the rubber sealing component stiffens, hardens, and becomes somewhat brittle. The lip of the oil seal eventually cracks because of the repeated bending and leads to oil seeping from a gear reducer. Once an oil seal is removed from a gear reducer, tiny cracks will be apparent.
Tiny cracks on lip of seal
Extremely brittle seal from overheating
|Probable Cause of Oil Leakage||What To Do|
|Too much oil causing leakage at vent plug.||Adjust oil level to correct level.|
|Clogged vent plug or no plug present causing elevated internal pressure.||Verify vent plug is installed and free from dirt and debris.|
|Verify vent plug is installed and free from dirt and debris.||Replace oil seal.|
|Loose hardware.||Check and tighten bolts where necessary.|
|Vent in wrong location.||Vent in wrong location.|
Loose shafting, while rare, does present itself from time-to-time. It is easily detected and normally can be seen visually. The shafting may be seen to move excessively either in an axial direction ‘in and out’ or in a radial direction ‘up and down’. If one or both of these conditions is detected, bearing service should be scheduled for the gear reducer.
Premature Bearing Failure
Insufficient or inadequate lubrication is absolutely the number one cause of the bearing failures we find during gear reducer inspections. Most enclosed gear reducers rely heavily on splash lubrication to provide cooling, flush contaminants, and maintain film thickness between mating moving parts. The tell-tale indication of lubrication failure on a tapered roller bearing in a gear reducer is when the retainer or ‘cage’ displays a ‘smeared’ appearance resulting from metal-to-metal contact.
‘Smeared’ or flattened retainer
Three typical issues that are encountered with chain drives include:
- Broken side plates.
- Chain elongation.
- Worn chain components.
Broken Side Plates
Cracking of side plates occur when a chain experiences loads in excess of the chain’s capacity. Sudden shock loads may also cause broken side plates.
Broken side plates
Often, roller chain appears to have ‘stretched’ and no longer fits well on the chain sprockets. The chain has actually not stretched but has experienced wear. Most of the wear occurs in the pins and bushings (the connecting members) since these are the components that articulate. Some wear is expected and normal. However, a good rule-of-thumb suggests replacing the chain when elongation reaches 3% more than its original length. A simple method to measure elongation, if expected, requires a straight section of the chain placed in tension and a 12” ruler. For simplicity, lets assume #40 pitch chain which measures ½” from pin to pin. Place the zero mark of the ruler in the center of a pin and count 20 pitches. The 10” mark on the ruler should align closely with the center of a pin (20 pitches x ½” = 10”). However, if the measurement is 10.3” or some, the chain would be recommended for replacement.
Worn Chain Components
Roller chain requires adequate lubrication. Without proper lubrication, the chain components rub together resulting in premature wear. Indications of inadequate lubrication include:
- Dry, dusty, and dirty chain.
- Worn or galled roller pins and bushings.
- Rusted link components.
The windings of electric motors are designed to withstand a certain amount of heat. When motors are operated under overload conditions, heat builds up inside the enclosure, the wire ties break away and the winding becomes burnt. When this occurs, the unit will not power up.