How to install bevel gears for peak performance

PRODUCT FOCUS: GEARING

How to install bevel gears
for peak performance

ROBERT F. WASILEWSKI, Arrow Gear Co.

You can’t put bevel gears together in the same way as spur and
helical types and get them to work well. They need to be assembled in

a specific way to ensure smooth running and optimum load
distribution between gears.

To optimize the performance of any technician share the responsibility for in- the three parameters on individual gears.
corporating this optimum positioning in Frequently, the manufacturer also
speed reducer, the gears must be ori- the gearbox. The designer must provide
ented to each other so that they run for ease of measurement, shimming, and marks a gearset number on each of the
smoothly without binding or interfer- assembly. And the technician must prop- two gears in a set. Gears manufactured in
ence. They must also maintain the cor- erly assemble the gears in the gearbox. sets should only be assembled with their
rect amount of tooth clearance (back- mates. Similarly, if one gear in a set fails,
lash) at all operating conditions. For spur Key assembly parameters the entire set should be replaced.
and helical gears, the best orientation
normally requires only that the center Several parameters contribute to Mounting distance. The distance from
distance and shaft alignment be correct proper assembly so that the gearbox op- a locating surface on the back of one gear
— no further adjustment is needed. erates smoothly and efficiently. The most to the centerline of a mating gear is
important are: called the mounting distance, Figure 1.
This is seldom the case with bevel This is the most important parameter for
gears, where mounting arrangements al- • Mounting distance. ensuring proper operation. The manufac-
low for wide variations in position. All • Matched teeth. turer establishes the optimum value for
types of bevel gears — straight, Zerol, spi- • Backlash. this distance by running the gearset and
ral, and hypoid — have an optimum posi- adjusting its position to obtain a tooth
tion for best performance. The manufac- Bevel gears of AGMA quality 8 or bet- contact pattern that is consistent with
turer determines this optimum position ter are normally manufactured and smooth running and optimum load distri-
by running tests of individual gearsets. tested in sets. Then, the manufacture bution between mating gear teeth. Be-
But, the gearbox designer and assembly marks the preferred values for each of cause of dimensional variations between
parts, each gear in a set has a unique
Mounting distance Gear value for the mounting distance and, in
for pinion centerline most cases, the manufacturer perma-
nently marks this value on each gear, Fig-
Pinion centerline Crossing point ure 2.

Mounting It is possible to manufacture all gears
distance to the nominal mounting distance speci-
fied on the drawing. But, the additional
for gear cost to do so is usually not warranted.

In cases where mounting distances
have not been marked on the gears, as-
sembly technicians must resort to a
costly trial and error method of marking
teeth with gear compound and adjusting

Back angles Robert F. Wasilewski is the design engineer-
Figure 1 — Mounting distances used for assembling bevel gears. ing manager for Arrow Gear Co., Downers
Grove, Ill.

POWER TRANSMISSION DESIGN s MARCH 1994 51

PRODUCT FOCUS: GEARING

the gears to obtain a suitable tooth con- of the two gears, preferably the pinion, fore adjusting backlash. This can be ac-
tact pattern. The lack of marked values at its marked mounting distance. The complished either by gaging or by direct
may also indicate that the gearset was reason is simple. Because bevel gears are measurement. Gages are used mostly for
not manufactured for high capacity or conical in shape, they can be assembled large gear production runs, and are based
consistency. in an almost infinite number of positions on the direct measurement method. For
(most of which cause poor performance) this reason, only the measurement
Matched teeth. After optimum tooth and still obtain the required backlash. method is described here.
contact is obtained in the running tests,
the manufacturer marks mating teeth in Frequently, a casual user assembles Note that when mating gears are ad-
engagement for identification. These bevel gears to obtain a specific amount of justed to their optimum position, their
marks usually consist of x’s or dots, Fig- backlash without regard to the mounting back angles, Figure 1, will probably not
ure 2, on two adjacent teeth of one gear distance. This is especially true for low be flush with each other. Do not attempt
and the mating tooth of the other gear. quality or lightly-loaded bevel gears. Al- to position bevel gears by making the
When assembling the gearset, position though this method occasionally works, it back angles flush.
the single marked tooth in the space be- is a risky approach where gears are
tween the two adjacent marked teeth. loaded to maximum capacity. It usually Measurement method. A typical gear-
causes shorter life and poor perfor- box, Figure 4, contains both an overhung
Backlash. The third most important mance. Only at the proper mounting dis- and a straddle-mounted gear. The proce-
parameter for a bevel gearset is the space tance will a gearset run correctly and still dure for assembling the bevel gears in this
between mating gear teeth, called back- have the right amount of backlash. gearbox by the measurement method is
lash. Unless otherwise specified, back- simple:
lash is measured normal (perpendicular) Gear assembly
to the tooth surface, Figure 3, and not in • On the housing, measure the distance
the plane of rotation. In most cases, the In most bevel gearsets, particularly
manufacturer marks the normal back- those with ratios above 2:1, the pinion po- from locating surface to bore centerline in
lash value on the gear, Figure 2. sition (mounting distance) effects tooth both horizontal and vertical directions
contact (the most important parameter (HMD and VMD).
Measurements in the plane of rotation, for good performance) to a larger extent
called transverse backlash, are as much than the gear position. Conversely, the • Record the gear and pinion mounting
as 40% larger, and should not be used for gear position has a larger effect on back-
assembly purposes. lash. For this reason, the manufacturer distances (MDG and MDP).
may have marked the mounting distance
#11444 FWIGhe2re mounting distances have been only on the pinion. In such cases, be sure • Measure the gear and pinion thick-
to accurately position the pinion accord-
marked on one or both gears, adjust these ing to its marked mounting distance be- nesses (WG and WP).
distances first. Then, use the normal
backlash value to verify proper assembly. X • Assemble the gears into their sub-
The backlash value should not be mea- XX
sured until after assembling at least one assemblies.

Figure 2 — Typical • Measure the length that controls gear
markings on bevel
gears include (left position in each subassembly (MAG and
to right) the gearset MAP).
number, mounting
• Calculate the distance from shim
distance (MD) in
inches for each mounting surface to crossing point for each
subassembly (MDV and MDH).
gear, x’s on mating
teeth, and the • Calculate the required shim thick-

backlash dimension nesses and assemble the gearbox with
(B/L) in inches. shims in place.

To aid the assembly technician, mark
the housing measurements on the housing
after it is machined and inspected.

Measurements involving the pinion and
gear are taken on the subassemblies, which
consist of gear, shaft, bearing, and gear
mounting components. This minimizes any
variations due to fits between these compo-
nents and greatly reduces the number of
measurements required. In this example,
only the gear thickness is measured before
putting together the subassembly. Sub-
tracting this distance from the mounting
distance lets the technician measure from
the front of the assembled gear.

52 POWER TRANSMISSION DESIGN s MARCH 1994

1444 FIG 3

Backlash
measurement

Figure 3 — Normal marked with a mounting distance, posi-
backlash in a set of tion the pinion at its mounting distance
spiral bevel gears is first, then adjust the gear position for the
measured perpen- required backlash.
dicular to the tooth
surface at the outer To obtain further verification, you can
perform an optional contact pattern
end (heel) of the check. This involves painting the tooth
tooth, using a surfaces on one gear with a thin marking
dial gage. compound and rotating the assembled
gears under light load. The compound
FIG 4 Normal Top view of teeth transfers from one gear to the other and
backlash shows how the teeth contact. Numerous
Pitch circle publications contain charts with typical
Verifying proper contact patterns (ANSI/AGMA 2008-B90
assembly. Use the Tooth outer end and ANSI/AGMA 2005-B88). The patterns
measurement seldom look exactly like the published ex-
method to assemble both gears, making gears at their mounting distances should amples and frequently require some in-
sure that the matched teeth are properly you measure backlash to verify proper as- terpretation.
engaged. Only after positioning both sembly. In cases where only the pinion is
Technicians familiar with contact pat-
Straddle mounted tern checking may believe that this tech-
gear subassembly nique is the preferred method of assem-
bly. However, the manufacturer has
already performed this check as part of
the testing (to determine mounting di-
mensions) and interpreted the contact

patterns so you don’t have to.s

Assembled gearbox

Shim thickness Figure 4 — Measurements
= MDV – VMD required for mounting both
overhung and straddle-mounted
MDV MAG WG MDG bevel gears in a typical
gearbox.

MDV = Shim thickness
MDG – WG + MAG = MDH – HMD

HMD MDH
VMD MAP

WP MDH = MDP – WP + MAP
MD
Housing
P

Overhung mounted

pinion subassembly

POWER TRANSMISSION DESIGN s MARCH 1994 53