Avoiding
Power Transmission Equipment Maintenance Errors
By Rodney Collier.
Here are some tips to prevent often-seen installation and maintenance
mistakes that lead to recurring downtime.
Too many companies have fallen victim to the shortcut of replacing
failed power transmission components rather than analyzing the root
cause of the failures.
For example, the operations manager for a supplier to the automotive
industry was quite frustrated late one Friday afternoon. He demanded
warranty for a failed gearbox that had lasted only two weeks. "The
previous unit lasted 1½ years," he reported. "This downtime is costing
our company $10K per day." When asked what caused the previous unit
to fail, he replied, "I’m not sure. All I know is that it failed
and someone on third shift replaced it with another gearbox."
I told him that whatever changed in the drive that caused the first
unit to fail had likelywreaked havoc on the current unit as well.
An inspection of the internal components of the gearbox likely would
have shown the gearbox was overloaded and that replacing it with
the same size unit would likely result in another failure.
This article identifies four frequently occurring installation
and maintenance errors involving power transmission equipment, and
provides guidelines to avoid the costly equipment downtime typically
associated with these errors.
Retension V-belt drives
The number one cause of V-belt failure is undertensioning. Undertensioned
drives allow a belt to move or slip independently of the
sheave. The resulting friction gives the belt’s sidewalls a shiny
or glazed appearance. Left unattended, this slippage creates heat
that hardens the belt’s rubber compound. The consequent flexing
required of the belt and this new hardened condition causes cracks
on the belt surface.
The belt may last a week or two, or it might make it a few months.
But ultimately, the belt will fail prematurely and usually at an
inopportune time. A slipping belt is easily identified by squealing
on startup, excessive heat at the driver or driven sheave, black
carbon dust underneath the drive, or glazed belt sidewalls.
The ideal tension for a belt is the least amount required to prevent
it from slipping under peak load conditions. There are several methods
of tensioning belts. Many installations are successful using a technician’s
experience or feel. However, the force deflection method
is more accurate. Belt manufacturers and suppliers typically provide
the force required to deflect a belt 1/64 in./in. of
belt center distance.
While it is important to follow the manufacturers’ recommendations
for installation of new belts, the typical error is that the tension
of a newly installed belt is not rechecked and adjusted after approximately
one shift of operation. I recommend the belt tension should be checked
after as little as 1 hr of operation. The need for retensioning
is based on the fact that belts have a tendency to seat or
find their home.
Paint removed from the grooves of new sheaves after only a few
minutes of operation shortens the center distance slightly. Some
belts may have "flashing" or excess material at certain spots which
wears off and allows the belt to ride deeper in the groove, resulting
in undertensioning. Additionally, as the belt warms up, it flexes
easier leaving more slack in the drive. If belts are not retensioned
after they have seated, slippage is imminent and premature failure
is likely.
Align shafts to equipment specs
The often-repeated mistake is taking the approach that if the coupling
halves fasten together, the drive is good to go. This is not necessarily
true. This approach often works for rigid couplings but not for
the widely used flexible types.
I recently worked with a client who was repeatedly breaking pump
shafts. The problem turned out to be the result of misaligned shafts.
The chosen coupling was flexible and relatively easy for mechanics
to assemble. The only drawback was they had to slide the motor out
of the way at every installation. The pump, on the other hand, could
not tolerate misalignment nearly as well as the coupling. In fact,
the bearing settings alone limit the amount of misalignment that
most equipment can tolerate. Proper shaft alignment would have saved
this company many hours of downtime, along with the associated labor
and equipment costs.
Aligning drives to coupling capabilities rather than equipment
capabilities often results in bending failure of the shaft(s). Preliminary
indications of misaligned shafts are excessive heat at the face
of the driver or driven equipment, oil seal leakage, and/or vibration.
The key is to ensure that shafts are aligned to within the equipment
specifications rather than the coupling capabilities. I recommend
achieving as near perfect angular and parallel alignment as possible—within
reason. Dial indicators, for example, are often used for aligning
shafts and are ideal for most applications. However, some critical
applications may require more sophisticated tools such as laser
alignment equipment. Regardless of the technique used, keep in mind
that if ideal alignment is not achieved, it may be the equipment
that suffers rather than the coupling.
No lubricants with tapered bore assemblies
One key benefit to using tapered bore products is that the design
allows for easy disassembly should the need arise. However, all
too frequently, installers apply an anti-friction lubricant to the
tapered bore or the barrel of the bushing. Lubricants of any kind
should never be used during installation of tapered bore products.
Assuming the installer is aware of recommended capscrew tightening
torques and practices those guidelines, an overtorqued condition
is likely because the friction between the mating parts has been
reduced by the lubricant and the capscrews are tricked into a larger-than-recommended
seated torque value. Under these conditions, the bushing is wedged
deeper than desired into the bore, escalating the hoop stress directed
at the hub. The result is typically seen in the form of fracture
through the drilled and tapped holes of the hub.
Again, lubricants should never be used in tapered bore products.
Torque the capscrews to recommended tightening torque and you will
be able to disassemble the components by using the drilled and tapped
back-off holes specifically designed into the components for ease
of product removal.
Analyze gearbox failure
Gear reducers fail for a reason. Ideally, they are properly sized
and maintained and live a long life, only failing as a result of
having reached their expected life span. Unfortunately, many times
this is not the case. Unpredictable overloading, worn accessory
components, and changes in the application often present undesirable
conditions for gear drives.
The weak link of a gearbox is almost always the gearing, the bearings,
or the shafting. Experience shows that often, it is the gearing
that fails. Therefore, when a gearbox fails prematurely, I always
ask a client what the condition of the gearing is. If the gearing
is damaged or has broken teeth, it is a safe deduction that the
unit has been subjected to loading beyond the mechanical limits
of the gearbox. This is, of course, assuming the unit was properly
lubricated. Replacement with another identical gearbox is likely
to produce similar results if the load requirements are not lessened
or the drive modified to within the mechanical limits of the gearbox.
Perhaps more is being asked of the drive in the form of more production
and consequent load increases. Perhaps something has failed elsewhere
in the drive train which causes more drag and requires the gearbox
to work harder. Whatever the case, when challenged to replace a
gearbox with failed gearing, evaluate the cause of failure closely
before automatically replacing the gearbox with a like unit.
When working with clients in trying to solve problems arising from
improper installation or maintenance of power transmission components,
I am frequently reminded of a comment a friend made to me 10 years
ago. He said, "Rod, there are mechanics and there are parts replacers."
My challenge to you is to follow proper installation and maintenance
techniques, analyze failures before replacing equipment, and enjoy
minimum downtime.
The writer has more than 15 years of experience in maintaining
power transmission products.
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