40 NEWEQUIPMENT.COM I NOVEMBER 2018
by Matt Nagle
From installation to troubleshooting, this comprehensive list of facts and tips
will help plants keep their equipment rolling smooth.
C A S When most people consider ball bearings, they envision robust and rugged machine parts that can take all kinds of knocking about, vibrations, high temperatures, and friction. But the reverse is true. Bearing reliability depends on many different factors such as loading, speed, ftting,
setting, operating temperature, and being kept in pristine conditions. Handling with
extreme care along with lubrication and maintenance to avoid contamination and
other environmental factors is also essential. Since rolling bearings are high precision
machine parts, they must be handled while taking the following main precautions into
account. Here are some tips to keep your equipment rolling.
1. KEEP BEARINGS AND SURROUNDINGS CLEAN
Dust and dirt—even if not visible—have harmful effects on bearings. Keep bearing handling
rooms as clean as possible, and do not remove bearings from their packaging until
just before use.
2. HANDLE WITH CARE
As heavy shocks during handling may
scratch or cause other damage to the bearing, possibly resulting in premature bearing
failure. Strong impacts can result in brinelling,
breaking, or cracking (see troubleshooting below). Never use a bearing that has been dropped,
as it may be dented and can cause a high level
of acoustic noise.
3. ALWAYS USE PROPER TOOLS
When handling bearings; avoiding general purpose tools. When applying lubricant,
never use an applicator that can leave contaminants behind, like a cotton swab, which
can leave fibrous particles.
4. PROTECT BEARINGS FROM CORROSION
This is the cause of 95% of bearing failures, so if this sounds like a broken record
when it comes to corrosion, that’s why. Even perspiration on the hands or various
other contaminants can cause corrosion, which is why wearing gloves is advisable
when handling bearings.
5. DETERMINE THE BEARING LUBRICANT REPLACEMENT/
Lubrication, whether oil or grease, will deteriorate over time and periodic replenishment is required. The replacement intervals depend on the operating conditions, type
of grease used, the magnitude of the bearing load, and the oil quantity. Temperature
can be measured from the housing outside surface, but a better way to do it is to obtain a direct reading from the bearing outer ring by a probe going through an oil hole.
Because the temperature tends to increase with operation, wait one to two hours after
the start of operation to monitor temperature.
For bearing noise, use a stethoscope or a proprietary instrument during operation
to investigate the volume and characteristics of bearing rotation noise. It is possible
to distinguish and diagnose bearing damage by its sound.
Vibration irregularities can be analyzed using a frequency spectrum analyzer to
measure the magnitude of the vibration and the distribution of the frequencies. Test
results can determine the likely cause of the bearing irregularity.
When bearings are used in
linear motion components,
there are specific issues to
watch out for.
When linear guides approach or reach their rolling
fatigue life, scaly particles
may start to flake away from
the surface of the raceways
and balls. This could be due
to improper assembly, poor
flatness of the mounting surface, or intrusion of foreign matter or water. If flaking is
noticed, there are a few steps to repair the issue. Improving the flatness of the mounting
surface or accuracy of the assembly could offer a solution. If lubrication is the culprit,
try switching to a better method or try a more suitable lubricant to see if that resolves
the issue. If there is contamination of any kind, review the protective covers and/or
enhance the sealing capability of the linear guide.
2. WEAR AND TEAR
Generally, this is a result of insufficient lubrication, improper installation or an intrusion of foreign matter. If misalignment isn’t a factor, checking the sealing capability
and the lubricant is the best option.
High energy impacts or improper handling of the product at its installation or in transit will easily cause this sort of damage. General dents on the raceway could also be
caused by contamination or intruded metallic particles. If this is suspected, improved
sealing and filtering of lubrication oil is advisable.
Corrosion on raceways and balls could be a result of water or corrosive substance
intrusion, inadequate lubrication, or improper storage. Review the sealing function and
the lubrication system, as well as the storing condition. It might make sense to cage
to a higher anti-acid material or better surface treatment.
5. FRETTING CORROSION
A specific type of wear caused by repeated rubbing between contacting surfaces.
It is almost certainly caused by completely dried out lubricant on the contacting area
because of minute amplitude oscillation. Use anti-fretting grease and distribute the
lubricant over the contact surfaces by longer stroke motions in certain frequency (i.e.:
once every 5,000 cycles).
6. ELECTROLYTIC CORROSION
This pit-like spot corrosion on balls and raceways is caused by an electric current,
creating sparks through a thin oil film between the contact points of the raceways
and balls, due to the electric potential difference between a rail and a ball slide. An
effective countermeasure is to provide an electric circuit, so that the current doesn’t
pass through the linear guides.
MATT NAGEL IS A SR. APPLICATIONS ENGINEER FOR NSK AMERICAS IN ANN ARBOR, MICH., SPECIALIZING IN
GEARBOXES AND OTHER INDUSTRIAL APPLICATIONS.
N S K A MERICAS