Maintenance Technology January 2016 - (Page 31)
Pitting Reveals Gear Issues
Best-practice maintenance varies
based on component type.
Neville Sachs, P.E.
UNDERSTANDING GEAR and reducer
metallurgies is the first step toward effective inspection and maintenance of these
components (see "Hardness, Lubrication
are Keys to Gear Inspection, p. 41, Maintenance Technology, Dec. 2015). Once that's
accomplished, the next step is to look at
the contact pattern of the teeth and rotate
the gear with the following in mind:
■ Does the pattern show across the entire
■ Is the pattern consistent all the way
around the gear? (It's usually a sign of
poor manufacturing quality control if it
A good contact pattern is critical to
good gear life. A contact pattern that runs
out at the end of a tooth means the tooth
load is a least twice the design load. If
the contact pattern only covers half of a
tooth, the stress at the heavily loaded edge
is more than four times the design load.
On through-hardened teeth, this may not
present much of a problem, as the gears
will wear in. On case-hardened teeth,
however, this reflects a very serious situation-one that can lead to tooth breakage.
Neville Sachs has spent many
years working in the field of
machinery reliability and lubrication for a wide range of industries. The
author of two books on failure analysis
and a contributor of sections to others,
he has also written more than 40
articles on these topics. A Registered
Professional Engineer, Sachs holds
STLE's CLS certification, among others.
Contact him at email@example.com.
gear teeth. If pitting
exists, it can tell you a
lot about gear
Pitted gear teeth?
With through-hardened gears, measure the amount of wear and use that to decide
when you need to replace the gears. If the gears don't involve human safety, run
them until the tooth thickness is reduced by 30% and don't worry about failure.
With case-hardened gears, any pitting is cause for concern. Very fine pitting
that looks like sandblasting, i.e., "micropitting," is indicative of very heavily
loaded teeth. If micropitting is consistent across the teeth, gear monitoring should
become a moderate priority. (A site's vibration analyst(s) should keep a close eye
on the tooth frequencies and sidebands.) Micropitting that's heavier on one side
indicates a misalignment between the mating gears and calls for further-immediate-investigation before something breaks. Is the housing distorted? Are the
components really misaligned?
Larger pitting, i.e., where individual pits are easily seen, indicates that the gears
are overloaded or the lubricant isn't doing its job-and requires very close attention. If there is a series of pits in a line across a case-hardened tooth, the risk of
tooth fracture increases. (Note: The presence of one or several large, isolated pits
almost always points to a manufacturing error.)
Important last words
■ Don't worry about backlash with gears always running in one direction.
■ If through-hardened gears are pitting, replace the lubricant with the next
higher viscosity grade. As long as the temperature doesn't increase substantially, the higher viscosity will result in a thicker lubricant film that leads to less
wear. To be sure you've improved the situation, before replacing an existing
lubricant, measure its temperature, then take the temperature of the higher
viscosity oil under similar operating conditions and compare the two on a
viscosity-temperature chart. MT
MAINTENANCETECHNOLOGY.COM | 31
Table of Contents for the Digital Edition of Maintenance Technology January 2016
On The Floor
Keeping Old Machines Running Like New
Operational Excellence Is A Competitive Necessity
Sell Reliability to Management
Several Hands Responsible For This Oil Debacle
Infrared Detects Leaks
Mobile CMMS Benefits
Gear Pitting Issues
Manage Energy Processes
Maintenance Technology January 2016