Previous tip of the month

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Find previous tips drawn from the monthly bulletin featuring short technical articles which had been produced by SKF Reliability specialists
January 2016: Bearing System Design
Bearing System Design

Did you know it’s possible for Rolling Element Bearings to have infinite life? 


Possible, but only achievable with great system design. Bearing numbers in catalogs only tell part of the story. Maximizing bearing service life in real-world applications requires key knowledge about bearings and the conditions under which they operate. SKF offers training which will help you to identify critical application design parameters so you can select the right bearing type, size and lubrication to ensure the longest possible Rolling Element Bearing life. 


However, once your application is in the field, things can change. Rolling Element Bearings are built into systems based on as much knowledge of the operating conditions as is available prior to locking in the design. 


Yet, when machines are repurposed in the field, you will need to pull out your system design tools again in order to ensure that the bearings can handle any potential changes that may occur. For example: common practice in the field is to use an electric motor in a vertical condition rather than horizontal, perhaps as a creative way to solve a space problem. It may be possible to simply replace the bearing(s) with another style to handle the new thrust load from the vertical shaft, while keeping the envelope dimensions the same. However, other concerns such as minimum loads should also be considered. SKF offers training and tools to solve these problems and more in order to maximize bearing service life.

February 2016: Turn the block over
Feb 2016_Tip of the month

Problem:

Bearing housings are cracking, or the bearings are not lasting as long as they should, so you initiate a Root Cause Analysis investigation to determine the reason for these failures. You find that the bearings and housings are the right size and type, there doesn’t seem to be anything wrong with what products you are using. Now it’s time to investigate the assembly and mounting procedures. You start to notice that the applications with early failure problems all use shim stock under the base to assure the proper base to shaft centerline height. Could there be a link?

Solution: 

It’s time to turn the housing over and look at the machined support surface on the bottom of the housing. If the machined surface goes all the way across the bottom, you need to shim the whole section, not just under the bolting feet.
Standard shims are made to be easy to use and come pre-cut with a slot for the bolt and a handle for you to use when inserting the stock, as shown below.
These shims are fine for most ball bearing pillow blocks, because they have a relief under the bearing to avoid stress risers in the cast iron, and only contact the pedestal at the two housing feet. But roller bearing housings tend to be machined all the way across the bottom due to the heavier loads these housings typically carry. The heavier load carrying capacity of roller bearings requires full support of the housing base. Consequently the base is fully machined and needs to be supported all the way across with either full-sized shims or ground spacer plates.
If a base like the one pictured on the right is only supported under the bolting feet with pre-cut shims, there will be excessive stress at the 6 o’clock position on both the bearing and the housing. Over time, the housing will react to this by cracking along the base, usually perpendicular to the boltto-bolt line. But this condition can affect the bearing immediately. The bearing may lose internal radial clearance, which can cause it to run hot. It can also experience two load zones, one at the bottom where it should be, and another at the top, where the bearing should run unloaded. All three conditions can compromise lubricant effectiveness, reducing bearing service life, even if you have the right lubricant.
Pay close attention to the bearing pedestal surface. The acceptable flatness tolerance is 0.002" per foot and 0.001" is preferred. Any gaps under the housing should be identified only after the complete housing has been fully and properly torqued to the mounting surface. Use a piece of 0.002" shim stock or a feeler blade to check for gaps and then shim and re-torque if necessary.
Never check a bearing housing for flatness on a flat inspection plate. The housing is clamped rigidly in a fixture for machining at the factory and when removed, internal stresses may cause the housing to distort slightly. When installed on a properly prepared flat surface, the housing will regain its machined dimensions. If shims are used to correct this potential distortion, the final shape and dimensions of the bore may not be correct and the bearing will not be fully supported.

Moral:

Something as simple and often overlooked as a piece of shim stock can cause premature housing and bearing failure, resulting in lost production, increased downtime and unnecessary maintenance expenses. So always remember, when mounting bearing housings and using shim stock: Turn the base over and look for the machined surface so you can shim appropriately. Prior to installation, check the bearing pedestal surface for any irregularities and for flatness. Machine as necessary to provide the best possible match between the pedestal and housing.

March 2016: Bearing lubrication: grease in vertical machines
Bearing lubrication: grease in vertical machines
Description:
You’re suffering repeated failures of grease-lubricated bearings in vertically mounted equipment, such as pumps, electric motors, mixers, and so on; especially in equipment that was originally designed for horizontal operation. The downtime is getting serious enough that management is starting to pay attention. When inspecting the bearings during an overhaul, shiny bearing raceways, surface distress and spalling are evident. What’s happening?

Solution:
There may be a lubrication problem. Gravity forces grease to flow down through the bearing, and leaks out of the arrangement. In vertical applications, grease with a higher consistency (e.g., NLGI 3) may be preferred to better retain the grease in the bearing. Due to the possible upward pumping effect of the bearing itself, grease with a lower consistency may be appropriate as well, provided it has high mechanical stability.
The suggested relubrication interval for vertical applications is half the interval required for a similar horizontal arrangement. After inspection of grease condition, the interval may be decreased or increased. Even better: re-grease continuously, with the desired grease quantity spread over the relubrication interval. Relubricate (e.g. via grease zirc) above the bearing. The use of a seal or grease retention shield is a prerequisite to prevent leakage. Operating temperatures and vibration also have a great effect on relubrication intervals.

Moral:
For grease-lubricated bearings that are mounted on vertical shafts, special attention is needed in grease selection, relubrication procedures and perhaps even sealing. For the right selection and intervals, consult the advisory program LubeSelect on the Knowledge Centre.
April 2016: Pocket-sized book saves BIG in the pocket
Apr 2016_1 TOM

Electric motors

The SKF Bearing Handbook for Electric Motors is available from the SKF Knowledge Centre or your local SKF representative. Ask for SKF Publication Number 140-430. Included in this gem are twelve great tips for avoiding common mistakes, which start on Page 2 of the Handbook:


Tip #6 says: “Pay attention to the bearing’s press fit. Use a press for any bearing under 4 in. O.D. Pressure should be applied only to the bearing ring with the press fit, which is usually the ring that rotates after the bearing is installed.”
There are many “whys” beneath the surface of these recommendations. A press fit of the inner ring of a bearing to a shaft literally stretches the ring like a rubber band, although on a very small scale.
Using proper mechanical tools such as a fitting tool evenly distributes mounting forces across the side face of the bearing when mounting the bearing. This is fine for smaller bearings up to 4 inches (100 mm) Outside Diameter (O.D.). The forces required to mount larger bearings could be great enough to either damage the shaft during mounting (adhesive wear) or to crack a ring since they are generally made of throughhardened steel. There is also a risk of personal injury if a chip cracks off during mounting. Don’t risk it.
For bearings larger than 4 inches (100 mm) O.D., use temperature mounting. Heat the bearing or cool the shaft to achieve a 150°F (~80°C) temperature difference between the bearing ring and the shaft. When mounting bearings with a press fit in the housing the same temperature difference is required. Place the wrapped bearing in the freezer and a shop lamp in the housing. Cover with a flameproof blanket to retain the heat. In 30 minutes or so the temperature difference will be safe for mounting. Alternatively, some housings can be heated with an induction heater if the heater has the proper capacity.

Heating a bearing with an induction heater is the safest, fastest method for shaft mounting larger bearings. Ensure that the heater has temperature controls to control heat and expansion and to prevent over heating. Additionally, only use heaters that provide an automatic demagnetization cycle.

May 2016: Alignment and soft foot
May 2016_1 TOM

Problem:
Machine feet perform many important functions. They constrain the machine for normal operating loads and in the event of a sudden failure such as a shaft lockup or a bearing seizure. Additionally, the feet are used to position the machine with respect to the other machine components in the train. The feet are also an important part of the machines structural or vibration dynamics.
A change in the bolted condition of the machine feet from the manufacturer’s recommendations can affect any or all of these important functions. The use of improper bolt grade, for example, may alter the strength of a foot joint and may not perform as designed in machine accident. Improper bolt torque techniques, damaged washers, and uneven bolting surfaces prevent precision machinery movements while aligning and create a constantly moving target. Changes in bolting stiffness due to incorrect clamping forces can alter machine/base stiffness and may induce resonances in a machine that used to perform well.
Notice in the photograph the uneven foot surface due to a rough casting and repeated bolt tightening. A soft washer was cut at one time in an attempt to fit the radius of the foot pad and is now so severely cupped that it is impossible to accurately position the machine horizontally.


Solution:
As part of any machine alignment or installation, always inspect the bolts and washers to ensure they are of the proper type and length and that they are in serviceable condition. Replace any soft washers with hardened washers. If the clearance hole in foot is enlarged, damaged, or rough, the use of a ground plate and/or machining of the foot surface will ensure the best possible clamping condition.
Last, and most important, always use a torque wrench with the proper sockets and adaptors when tightening machine feet. Follow a tightening sequence and if all of the softfoot has been removed in advance, the machine will move predictably through the alignment remain in position when placed in service.

June 2016: Bearing Maintenance and Technology
Jun 2016_TOM

Am I using the wrong tool with bearings?

How do you know if you’re using the wrong tool to mount or dismount a ball or roller bearing?
Simple check: if you have to reach for a hammer, chisel, punch, screwdriver or torch to install or remove a bearing, you’re generally on the wrong track, and you might actually get hurt.


Here are the details:
Hammer and/or chisel:
applying force directly to bearing parts can cause chipping and cracking of the through hardened steel. Too many people have been injured, some seriously, to take this risk.
Hammer and punch:
only proper for mounting an eccentric locking collar-type bearing. Locking nuts will be damaged and chipped out with this combination of tools.
Screwdriver:
often used to remove bearing seals or shields. This tactic will not improve bearing lubrication! If your bearing isn’t performing, consult SKF for help.
Torch:
on installation, you risk seriously overheating the bearing, losing hardness of the steel which will cause premature failure. On removal, a torch can crack rings (see above) or permanently bend the shaft you’re working on.

July 2016: Prevent Roller Bearing Damage on Startup
Tapered roller and spherical roller thrust bearings
Tapered roller and spherical roller thrust bearings have internal flanges that must be considered during mounting.

Ensure proper roller seating during assembly:

The rollers of both bearings have a contact angle that can allow the rollers to drop out of contact with their guiding flanges (red areas in figures) during assembly. To ensure proper roller seating during assembly, rotate the shaft or the outer ring (common in wheel applications). This rotation will ensure proper contact of the large end of the rollers against the bearing internal flanges which is a prerequisite before measuring any end play. (NOTE: The green arrows in the figures at right indicate the direction of roller seating.)


Consequences if not done properly:

If rotation is omitted, smaller end play values will be measured as the rollers have not been properly seated. During startup, excess end play may allow rollers to skew so
severely that they may skid or slip instead of rolling properly. This skidding can result in very early failure of the bearing from extremely high heat generation, lubricant failure, and cage damage that could seize the bearing. Seizure may occur in the first minute of operation! 


Guidance:

Follow manufacturer’s end play recommendations during assembly. If guidance isn’t available, contact SKF Applications Engineering Service for assistance or fill in your request on the right hand side "Contact us".

August 2016: Vibration analysis: resonance
Traditional Power
Description:
High vibration at one times running speed was noted on the exciter end of an 18 MW, 2-pole Gas Turbine Generator. The vibration amplitude was highest in the axial direction, and had been a problem for 3 years.

During a pre-analysis meeting, the first step in a 6-step analysis process, it was found that the generator bearing had been replaced multiple times during the preceding three years, with little or no effect. Following recommendations of the OEM and several vibration specialists, the unit’s alignment was checked and adjusted three different times, again with little or no effect. These unsuccessful correction attempts cost the company approximately $500,000 over a three year period.
Analysis of the machine during start-up showed a large increase in the vibration amplitude beginning at 3550 RPM on up to normal operating speed of 3600 RPM. These vibration levels were observed without the field being applied, eliminating any generator/electrical problems as contributors to the problem.
During coast down, a 900 phase shift was observed within a very small change in speed. These two symptoms:
1. Large change in amplitude with a small change in speed
2. 900 phase shift during coast down are very strong indicators of aresonant condition.

Solution:
A vibration survey was conducted on the exciter end of the generator to locate the resonance. Because of interference from the collector ring covers the resonance could not be located. The unit was shut down and the covers were removed, and with appropriate safety precautions, the generator was restarted. The resonant condition now occurred at 3400 RPM. The vibration amplitudes at running speed, 3600 RPM, returned to the low values previously observed prior to the sudden increase three years earlier.
The combination of mass and stiffness of a mechanical structure determine its natural frequencies. Increasing mass decreases a natural frequency and adding stiffness increases a natural frequency. Removing these covers reduced both mass and stiffness. Since the resonant frequency decreased, the loss of the stiffness of the covers obviously had a more significant impact than that of the decreased mass.
At this time maintenance recalled removing a 1/2" rubber gasket three years earlier during a routine cleaning and replaced it with standard gasket material. The rubber gasket was originally installed by the OEM to decrease the stiffness of the exciter assembly.  By replacing the rubber gasket with standard gasket material, the natural frequency of the exciter assembly was raised to near the normal operating speed.
A resonance was created that amplified the generator vibration to its high levels. Replacing the hand made gasket with the 1/2” rubber gasket returned the natural frequency to its correct value. The unit then ran successfully. Further improvement was achieved by performing a balance on the generator rotor.

Moral:
Resonance is often the least understood characteristic of vibration, yet is estimated that 20% or more of machines are affected to a degree by resonance. Since resonance is an amplifier, it creates confusing symptoms in the vibration spectrum, resulting in frequent and reoccurring misanalysis using normal vibration techniques. Identifying resonance can be relatively easy using practical techniques.
This case history also emphasizes the importance of determining the proper and full history of the problem.  Seemingly small changes can occasionally have a large impact on a machine’s operation. These problems can be easily solved with a systematic approach. “Non-vibration specialist” technicians solved this problem, after attending the CM 103 Machinery Inspection & Evaluation course.
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