Gears and Gearboxes 309
Where:
TF = tangential force
S
TF
= separating force
T
TF
= thrust force
hp = input horsepower to pinion or gear
D
p
= pitch diameter of pinion or gear
rpm = speed of pinion or gear
φ = pinion or gear tooth pressure angle
λ = pinion or gear helix angle
Herringbone Gears
Commonly called “double helical” because they have teeth cut with right and
left helix angles, they are used for heavy loads at medium to high speeds.
They do not have the inherent thrust forces that are present in helical gear
sets. Herringbone gears, by design, cancel the axial loads associated with a
single helical gear. The typical loads associated with herringbone gear sets
are the radial side-load created by gear mesh pressure and a tangential force
in the direction of rotation.
Internal Gears
Internal gears can only be run with an external gear of the same type, pitch,
and pressure angle. The preload and induced load will depend on the type
of gears used. Refer to spur or helical for axial and radial forces.
Troubleshooting
One of the primary causes of gear failure is the fact that, with few exceptions,
gear sets are designed for operation in one direction only. Failure is often

Gear set not suitable for
application
• • • •
Gears mounted backward on shafts • • •
Incorrect center-to-center distance
between shafts
• •
Incorrect direction of rotation • • •
Lack of or improper lubrication • • • • • • •
Misalignment of gears or gearbox • • • • • •
Overload • • • • •
Process induced misalignment • • • •
Unstable foundation • • • •
Water or chemicals in gearbox •
Worn bearing • •
Worn couplings •
Source: Integrated Systems Inc.
Gears and Gearboxes 311
practice permits longer operation times, the torsional power generated by
a reversed gear set is not as uniform and consistent as when the gears are
properly installed.
Gear overload is another leading cause of failure. In some instances, the
overload is constant, which is an indication that the gearbox is not suitable
for the application. In other cases, the overload is intermittent and only
occurs when the speed changes or when specific production demands cause
a momentary spike in the torsional load requirement of the gearbox.
Misalignment, both real and induced, is also a primary root cause of gear
failure. The only way to assure that gears are properly aligned is to “hard
blue” the gears immediately following installation. After the gears have run
for a short time, their wear pattern should be visually inspected. If the

potential failure modes for the gearbox.
Abrasion
Abrasion creates unique wear patterns on the teeth. The pattern varies,
depending on the type of abrasion and its specific forcing function.
Figure 14.36 illustrates severe abrasive wear caused by particulates in the
lubricating oil. Note the score marks that run from the root to the tip of the
gear teeth.
Gears and Gearboxes 313
Figure 14.37 Pattern caused by corrosive attack on gear teeth
Figure 14.38 Pitting caused by gear overloading
Chemical Attack or Corrosion
Water and other foreign substances in the lubricating oil supply also cause
gear degradation and premature failure. Figure 14.37 illustrates a typical
wear pattern on gears caused by this failure mode.
Overloading
The wear patterns generated by excessive gear loading vary, but all share
similar components. Figure 14.38 illustrates pitting caused by excessive tor-
sional loading. The pits are created by the implosion of lubricating oil. Other
wear patterns, such as spalling and burning, can also help to identify specific
forcing functions or root causes of gear failure.
15 Hydraulics
“Only Permanent Repairs Made Here”
Hydraulic Knowledge
People say knowledge is power. This is true in hydraulic maintenance. Many
maintenance organizations do not know what their maintenance personnel
should know. We believe in an industrial maintenance organization where
we should divide the hydraulic skill necessary into two groups. One is
the hydraulic troubleshooter; they must be your experts in maintenance,
and this should be as a rule of thumb 10% or less of your maintenance
workforce. The other 90% plus would be your general hydraulic main-

●
Null a servo valve
●
Troubleshoot a hydraulic system and utilize “Root Cause Failure Analysis”
●
Replace any system component to manufacturer’s specification.
●
Develop a PM program for a hydraulic system.
●
Flush a hydraulic system after a major component failure
General Maintenance Person
Knowledge:
●
Filters (function, application, installation techniques)
●
Reservoirs (function, application)
●
Basic hydraulic system operation
●
Cleaning of hydraulic systems
●
Hydraulic lubrication principles
●
Proper PM techniques for hydraulics
Skill:
●
Change a hydraulic filter and other system components
●
Clean a hydraulic reservoir
●

samples.
1. Pressure filter:
Pressure filters come in
collapsible and
noncollapsible types.
The preferred filter is
the noncollapsible type.
Remove the old
filter with clean
hands and install
new filter into the
filter housing or
screw into place.
Least preferred:
Based on
equipment
manufacture’s
recommen-
dations.
2. Return filter:
Typically has a bypass,
which will allow
contaminated oil to
bypass the filter before
indicating the filter
needs to be changed.
CAUTION: NEVER
allow your hand
to touch a filter
cartridge. Open the

Table 15.1 continued
Component
Component knowledge Best practices Frequency
Hydraulic
reservoir
A reservoir is used to:
Remove contamination.
Dissipate heat from the
fluid. Store a volume of
oil.
Clean the outside of
the reservoir to
include the area
under and around
the reservoir.
Remove the oil by a
filter pump into a
clean container,
which has not had
other types of fluid
in it before.
Clean the insides of
the reservoir by
opening the
reservoir and
cleaning the
reservoir with a
lint-free rag.
Afterward, spray
clean hydraulic fluid

troubleshoot a system
problem quickly.
Check and record
flow and pressure
during specific
operating cycles.
Review graphs of
pressure and flow.
Check for excessive
fluctuation of the
hydraulic system.
(Designate the
fluctuation
allowed.)
Pressure checks:
Preferred: daily
Least preferred:
Weekly
Flow & pressure
checks:
Preferred: two
weeks
Least preferred:
monthly
318 Hydraulics
Root Cause Failure Analysis
As in any proactive maintenance organization you must perform Root Cause
Failure Analysis in order to eliminate future component failures. Most
maintenance problems or failures will repeat themselves without someone
identifying what caused the failure and proactively eliminating it. A pre-

Hydraulics 319
●
Does the system operate at maximum flow and pressure 70% or better
during operation?
●
Is the system located in a dirty or hot environment?
Second: What requirements does the equipment manufacturer state for
preventive maintenance on the hydraulic system?
Third: What requirements and operating parameters does the component
manufacturer state concerning the hydraulic fluid ISO particulate?
Fourth: What requirements and operating parameters does the filter
company state concerning its filters’ ability to meet this requirement?
Fifth: What equipment history is available to verify the above procedures for
the hydraulic system?
As in all preventive maintenance programs, we must write procedures
required for each PM task. Steps or procedures must be written for each
task, and they must be accurate and understandable by all maintenance
personnel from entry level to master.
Preventive maintenance procedures must be a part of the PM job plan, which
includes (see Figure 15.1):
●
Tools or special equipment required for performing the task;
●
Parts or material required for performing the procedure with store room
number;
●
Safety precautions for this procedure;
●
Environmental concerns or potential hazards.
A list of preventive maintenance tasks for a hydraulic system could be:

3. Oil Filter Pump
SAFETY PRECAUTIONS:
1. Observe plant and area specific safe work practices.
MAINTENANCE PROCEDURE:
1. Inspect hydraulic oil reserve tank level as follows:
a) If equipped with sight glass, verify oil level at the full mark. Add oil as required.
b) If not equipped with sight glass, remove fill plug/cap.
c) Using flashlight, verify that oil is at proper level in tank. Add oil as required.
2. Record discrepancies or unacceptable conditions in comments.
PM Procedure Courtesy of Life Cycle Engineering, Inc.
Figure 15.1 Sample preventive maintenance procedure
●
Check and record pump flow.
●
Check hydraulic hoses, tubing, and fittings.
●
Check and record voltage reading to proportional or servo valves.
●
Check and record vacuum on the suction side of the pump.
●
Check and record amperage on the main pump motor.
●
Check machine cycle time and record.
Hydraulics 321
Preventive maintenance is the core support that a hydraulic system must
have in order to maximize component life and reduce system failure.
Preventive maintenance procedures that are properly written and followed
properly will allow equipment to operate to its full potential and life cycle.
Preventive maintenance allows a maintenance department to control a
hydraulic system rather than the system controlling the maintenance depart-

Parts and material cost?
●
Labor cost?
322 Hydraulics
●
Production downtime cost?
●
Any other cost you may know that can be associated with a hydraulic
system failure?
2 Track hydraulic system fluid analysis. Track the following from the results
(taking samples once a month):
●
Copper content
●
Silicon content
●
H
2
O
●
Iron content
●
ISO particulate count
●
Fluid condition (viscosity, additives, and oxidation)
When the tracking process begins, you need to trend the information that
can be trended. This allows management the ability to identify trends that
can lead to positive or negative consequences. See Figure 15.2.
123456789101112
0

tamination that is introduced into an existing hydraulic system through the
addition of new fluid and the device used to add oil to the system.
Additional information: Hydraulic fluid from the distributor is usually not
filtered to the requirements of an operating hydraulic system. Typically, this
oil is strained to a mesh rating and not a micron rating. How clean is clean?
Typically, hydraulic fluid must be filtered to 10 microns absolute or less for
most hydraulic systems; 25 microns is the size of a white blood cell, and
40 microns is the lower limit of visibility with the unaided eye.
Many maintenance organizations add hydraulic fluid to a system through a
contaminated funnel and may even, without cleaning it, use a bucket that
has had other types of fluids and lubricants in it previously.
Recommended equipment and parts:
●
Portable filter pump with a filter rating of 3 microns absolute.
●
Quick disconnects that meet or exceed the flow rating of the portable
filter pump.
●
A
3
4
" pipe long enough to reach the bottom of the hydraulic container
your fluids are delivered in from the distributor.
324 Hydraulics
●
A 2" reducer bushing to
3
4
" NPT to fit into the 55-gallon drum, if you
receive your fluid by the drum. Otherwise, mount the filter buggy to the

Air breather
and filter
Pump
inlet
line
Drain
return
Return line
Sealed flange
Figure 15.4 Hydraulic reservoir modification
Objective: The objective is to eliminate the introduction of contamina-
tion through oil being added to the system or contaminants being added
through the air intake of the reservoir. A valve needs to be installed for oil
sampling.
Additional Information: The air breather strainer should be replaced with a
10-micron filter if the hydraulic reservoir cycles. A quick disconnect should
be installed on the bottom of the hydraulic unit and at the
3
4
level point on
the reservoir with valves to isolate the quick disconnects in case of failure.
This allows the oil to add from a filter pump as previously discussed and
would allow for external filtering of the hydraulic reservoir oil if needed.
Install a petcock valve on the front of the reservoir, which will be used for
consistent oil sampling.
Equipment and parts needed:
●
Quick disconnects that meet or exceed the flow rating of the portable
filter pump
●

slowing down a rotating wheel on a vehicle, or a piston sliding in a cylinder.
In sliding friction, because the contact pressure is usually spread over a large
area, the pressure per square inch is relatively low.
Rolling Friction
Rolling friction takes place when a spherical or cylindrical body rolls over a
surface. Common examples of rolling friction are ball and roller bearings.
With ball or roller friction bearings the area of contact is quite small; how-
ever, the pressure loading, or pressure per square inch, is high. There is
also a very small amount of sliding friction between the ball or roller and
328 Lubrication
the separators because the components are rolling instead of sliding as in
the piston example above.
With gears, both sliding and rolling conditions exist as the gears mesh and
unmesh. They are grouped according to their contact area and action.
Fluid Friction
Fluid friction refers to air, water, or other types of fluid providing the resis-
tance to movement between two objects. One example of fluid friction
would be the resistance of air to an airplane flying. Another example would
be the torque converter in an automatic transmission; the transmission
fluid provides the power to drive the automobile through friction with the
impeller blades.
Lubrication Theory
When lubricating oil is applied to each of the component surfaces, a thin
film of oil is formed, filling up the depressions and covering the projections.
Due to the film of oil between the two surfaces, sliding, not friction, will
occur. This condition is called fluid lubrication. See Figure 16.1.
In theory, the oil forms in layers of globules, one layer adhering to each metal
surface and any number of layers of globules in between. (See Figure 16.2.)
In the illustrations, layer (1) adheres to the top surface, layer (9) in Figure
16.2(a) or layer (8) in Figure 16.2(b) adheres to the bottom surface, and

Shaft center
Oil delivery
Figure 16.4 Rolling action
The turning shaft has been likened to a pump forcing oil between shaft and
bearing, with hydraulic pressure creating an oil wedge to force the shaft
against the opposite side.
It should be noted that this theory depends on a satisfactory supply of oil
to form a continuous film. Lack of oil after the rotation begins means that a
lubricating film and wedge cannot be established, and the metal-to-metal
Lubrication 331
Bearing center
Shaft center
Oil delivery
Figure 16.5 Establishment of fluid film
contact will be maintained, generating heat and eventually wearing out the
bearing.
In Figure 16.6, the area marked C is the point of high pressure, and
the oil film is thinnest in that area. Oil should come in from the top of
low-pressure area, where it can be picked up by the shaft, and brought
around to the high-pressure area.
When rotation starts, the coefficient of friction is quite high, but as soon as
the shaft has made about half a turn, or enough to form a film of oil with
the bearing, the coefficient of friction drops to a low level.
In an antifriction bearing there are two oil wedge formations due to the
three-unit construction of the bearing.
Ball Bearing Oil Wedge Formation
●
Outer race
●
Ball

Heat from operation is usually in a very small range, but in some machines
an allowable rise of 100
◦
F is predicted. Heat from surroundings will vary,
from an exposed bearing in winter to a bearing next to a large boiler. The
temperature range could be as much as 150
◦
F.
Properties of Oil
Viscosity
A lubricant for any machine must meet the requirements set by critical
load, speed, and temperature. The correct lubricating oil is selected for
Lubrication 333
its physical properties of viscosity and pour point, plus the extra qualities
obtained by additives or special agents. Lubricating oil is used to minimize
wear, heat rise, and power loss due to friction, to act as a cushion to absorb
shock and vibration, and to act as a cleansing agent by washing away minute
wear particles.
Viscosity ratings are obtained by a viscometer that measures the amount
of time it takes for a measured amount of oil to flow through a measured
opening at a definite temperature. (Saybolt Universal Viscosity [SUS] is the
time in seconds for 60 cubic centimeters of a fluid to flow through the
orifice of the Standard Saybolt Viscometer at a given temperature under
specified conditions.) Temperatures taken are 100
◦
F and 210
◦
F (100
◦
, 130

◦
–200
◦
Below 150 42 65 150
150–300 42 65 120
300–750 42 50 65
Over 750 42 50 55