Center Height, Bolt Pattern and Shaft Diameter: How to Measure a Gearbox for Replacement
When an industrial gearbox needs replacement, three measurements can quickly determine whether a proposed reducer has any chance of fitting the existing machine:
Center height. Bolt pattern. Shaft diameter.
Get one of them wrong and a gearbox that looked like a replacement on paper may require:
a new mounting plate,
coupling changes,
shaft modifications,
machine realignment,
fabrication or
an entirely different reducer.
This becomes especially important when the original gearbox is obsolete, the nameplate is unreadable, or the manufacturer no longer offers an exact replacement.
Fortunately, maintenance teams can collect many of the most useful gearbox replacement measurements directly in the field.
This guide explains how to measure an industrial gearbox for replacement, what tools to use, where common measuring mistakes occur, and what additional dimensions should be recorded before requesting a replacement quote.
Before Measuring: Make the Equipment Safe
Never measure a gearbox around exposed moving equipment.
Before taking measurements that require access to the drive system, follow your facility's hazardous-energy-control procedures and applicable lockout/tagout requirements.
The gearbox, motor, coupling, driven equipment, and other energy sources may need to be isolated before guards are removed or personnel enter hazardous areas.
Do not reach around:
rotating shafts,
couplings,
belts,
chains,
sprockets or
other moving equipment
to obtain a measurement.
Accurate dimensions are important.
Safe access comes first.
What Tools Should You Bring?
A useful gearbox measurement kit can include:
tape measure,
digital or dial caliper,
micrometer where greater accuracy is needed,
steel rule,
straightedge,
combination square,
flashlight,
inspection mirror,
marker,
notebook or tablet,
camera or smartphone and
manufacturer's drawings if available.
For larger equipment, additional measuring tools may be useful.
The tool should match the accuracy required.
A tape measure may be sufficient for overall housing length.
It is generally not the best tool for determining whether a shaft is:
3.000 inches
or
75 millimeters.
Use a caliper or micrometer for shaft measurements.
First, Photograph Everything
Before measuring, take photographs.
Capture:
complete gearbox,
gearbox nameplate,
motor nameplate,
output connection,
input connection,
mounting base,
coupling,
driven equipment,
backstop,
torque arm,
cooling equipment and
surrounding clearances.
Take both wide and close-up photographs.
The photographs provide context for the dimensions later.
A spreadsheet entry saying:
Center height = 10.5 in.
is useful.
A photograph showing exactly which gearbox and mounting surface were measured makes it much more useful.
Record the Nameplate Before Taking Dimensions
If the nameplate is readable, record:
manufacturer,
complete model number,
serial number,
ratio,
input speed,
output speed,
horsepower or kW and
other listed information.
Do not assume measurements replace nameplate information.
The best replacement request combines:
identification + application data + dimensions + photographs.
If the gearbox model can still be identified, a supplier may be able to obtain an original dimensional drawing.
That is usually preferable to reconstructing every dimension manually.
Measurement 1: Shaft Center Height
For a foot-mounted gearbox, shaft center height is one of the most important replacement dimensions.
It is the vertical distance between the gearbox mounting surface and the centerline of the shaft.
For replacement purposes, the output-shaft center height is often particularly important.
Why Center Height Matters
Imagine the existing gearbox has:
Output center height = 10.00 inches
and the proposed gearbox has:
Output center height = 11.25 inches.
The new output shaft will sit 1.25 inches above the existing driven equipment.
The gearbox may have:
the correct ratio,
adequate torque and
the correct shaft diameter,
but the shafts will not align.
The plant may need to:
modify the base,
reposition equipment,
redesign the coupling arrangement or
choose another gearbox.
That is why center height is often one of the first dimensions worth comparing.
How to Measure Gearbox Center Height
The desired measurement is:
Mounting surface → center of shaft
The challenge is finding the exact shaft center.
One practical method is to measure:
from the mounting surface to the bottom of the shaft, then
add half the shaft diameter.
For example:
Distance from mounting surface to bottom of shaft:
8.50 in.
Shaft diameter:
3.00 in.
Half the shaft diameter:
1.50 in.
Therefore:
Center height = 8.50 + 1.50 = 10.00 in.
This can be easier than trying to visually estimate the shaft centerline.
Use the Correct Mounting Surface
Be careful about your starting point.
Measure from the surface that actually establishes the gearbox installation height.
That may be:
gearbox foot,
mounting plate,
machined base or
another defined mounting surface.
Do not accidentally include:
temporary shims,
debris,
unrelated structural members or
another surface
without documenting it.
If shims are installed, record them separately.
Measurement 2: Bolt Pattern
The mounting bolt pattern determines whether the replacement can attach to the existing base.
For a typical foot-mounted gearbox, you may need:
front-to-rear bolt spacing,
side-to-side bolt spacing,
mounting-hole diameter and
overall foot dimensions.
Measure Center-to-Center
Bolt patterns should normally be documented center-to-center.
Do not measure from the outside edge of one bolt to the outside edge of another and call that the bolt spacing.
What matters is the relationship between the hole centers.
For example:
Side-to-side bolt centers: 14.00 in.
Front-to-rear bolt centers: 22.00 in.
That provides much more useful information for comparing a replacement drawing.
How to Measure Center-to-Center Bolt Spacing
If the bolt or hole centers are easy to identify, measure directly from center to center.
If they are not, another method can be used.
For two equal-diameter holes, you can measure from the same corresponding edge of one hole to the same edge of the other.
For example:
Left edge of Hole A → left edge of Hole B
This gives the same center-to-center distance when the holes are equal in diameter and the measurement is made accurately.
A caliper can be useful on smaller mounting patterns.
For large gearbox bases, a tape measure or steel rule may be more practical.
Measure Both Directions
Do not record only one bolt spacing.
For a four-foot mounting pattern, document both:
Longitudinal spacing
Along the length of the gearbox.
Transverse spacing
Across the gearbox.
Also check whether all mounting holes form a simple rectangle.
Some gearbox bases have:
additional holes,
offset patterns,
slotted holes or
different mounting configurations.
Photograph the base.
Record Mounting-Hole Diameter
The replacement gearbox may have the same bolt spacing but use a different mounting-hole diameter.
Measure or identify:
hole diameter,
bolt size and
whether holes are round or slotted.
If bolts remain installed and the hole itself cannot be measured, record the bolt information and mark the actual hole diameter as unverified.
Do not guess.
Measurement 3: Output-Shaft Diameter
Output-shaft diameter affects compatibility with:
coupling hubs,
sprockets,
pulleys,
sheaves,
bushings and
other driven components.
This measurement should be taken accurately.
Use a Caliper or Micrometer
Do not estimate shaft diameter with a tape measure if accurate replacement information is required.
A shaft that appears to be approximately three inches could be:
3.000 in.,
2.938 in.,
75 mm,
80 mm or
another dimension.
Those differences matter.
Measure across a clean, undamaged portion of the shaft.
If possible, take measurements in more than one location and orientation.
Wear, corrosion, coatings, or damage can distort the reading.
Inch or Metric?
Never assume.
A 75 mm shaft equals approximately:
2.953 inches.
That may look like a three-inch shaft during a quick inspection.
But a standard 3.000-inch coupling bore will not represent the same fit as a 75 mm bore.
Record the dimension with units:
75.00 mm
not:
approximately 3 inches.
This is especially important on equipment that combines American and European components.
Measurement 4: Output-Shaft Extension
Shaft diameter alone is insufficient.
Measure how far the shaft extends from the gearbox reference surface.
A replacement shaft can have the correct diameter but be:
too short,
too long or
positioned differently.
Record:
Shaft extension = [dimension]
and identify the reference point used.
A photograph with an arrow showing the measured dimension is even better.
Measurement 5: Output-Shaft Keyway
Measure and document the keyway.
Record:
key width,
key height,
usable keyway length,
keyway position and
key dimensions.
Do not assume two shafts of the same diameter automatically use identical keys.
Metric and inch key standards can create additional differences.
If the existing coupling hub or sprocket will be reused, keyway compatibility becomes particularly important.
Measurement 6: Shaft Shoulder Location
Look at the shaft geometry.
Does the shaft have:
a shoulder,
retaining ring,
threaded end,
tapped hole,
spacer or
other locating feature?
Measure where the driven component sits relative to the gearbox.
This can determine whether the existing coupling or sprocket can be positioned correctly on the replacement.
Measurement 7: Input-Shaft Diameter
If the gearbox is connected to the motor through a separate coupling, measure the input shaft too.
Record:
diameter,
extension,
keyway and
key.
Then record the motor shaft.
This helps determine whether the existing coupling can be reused.
Measurement 8: Input-Shaft Center Height and Position
If the motor and gearbox are mounted on a common base, document the gearbox input-shaft position.
Measure:
input center height,
horizontal position and
shaft-end location.
A replacement gearbox with a different input location may require the motor to move.
That may lead to:
new base holes,
coupling changes,
shimming,
guard changes and
realignment.
Measurement 9: Distance Between Shaft Ends
For coupled equipment, document the relationship between the gearbox shaft and mating shaft.
Depending on the coupling, you may need to know:
shaft-end separation,
coupling hub positions,
spacer length and
available axial adjustment.
This becomes especially important with spacer couplings.
A new gearbox may have the correct shaft diameter and centerline while positioning the shaft end differently.
Measurement 10: Overall Base Length and Width
Measure the gearbox mounting feet or base.
Record:
base length,
base width and
individual foot dimensions where useful.
This helps determine whether the new gearbox will physically sit on the existing structure.
A replacement can have a compatible bolt pattern while still having a housing or foot that interferes with nearby equipment.
Measurement 11: Overall Length, Width and Height
Document the installation envelope.
Measure:
Length
Including relevant shaft projections and accessories.
Width
Including fans, backstops, shafts, and other projections.
Height
From mounting surface to the highest relevant point.
The objective is to answer:
Will the replacement physically fit in the available space?
Consider nearby:
structures,
guards,
piping,
motors,
electrical equipment,
walkways and
other machinery.
Measurement 12: Flange Dimensions
If the gearbox is flange mounted, the flange may be more important than a foot-mount bolt pattern.
Record:
outside flange diameter,
pilot diameter,
bolt-circle diameter,
number of holes,
hole diameter,
flange thickness and
shaft relationship to the flange.
The pilot diameter can be particularly important because it may locate the gearbox concentrically with the driven machine.
Measurement 13: Hollow-Shaft Bore
For shaft-mounted reducers, measure or identify:
hollow-shaft bore,
bore length,
keyway,
driven-shaft diameter,
bushing and
shrink disc where applicable.
Do not assume a visually similar shaft-mounted gearbox will fit the existing driven shaft.
Obtain manufacturer documentation whenever possible.
Measurement 14: Torque-Arm Location
For shaft-mounted reducers, document the torque arm.
Record:
gearbox attachment location,
arm length,
hole diameter,
angle,
connection point and
surrounding structure.
The replacement may fit the driven shaft but use a different torque-arm geometry.
That can require a new bracket.
Measurement 15: Sprocket or Sheave Position
If the gearbox drives through a sprocket, pulley, or sheave, record where it sits on the output shaft.
Measure from a known gearbox reference point to the centerline of the driven component.
Why?
Because moving a sprocket farther out on the shaft can increase overhung load.
A replacement gearbox must not only accommodate the component physically—it must also have adequate shaft and bearing capacity for the resulting load.
Measurement 16: Accessory Clearances
Look beyond the basic housing.
Measure clearance for:
backstop,
cooling fan,
breather,
oil pump,
filter,
heat exchanger,
torque arm,
inspection covers and
lifting points.
A replacement housing may fit perfectly while an accessory collides with the machine frame.
Create a Field Measurement Sheet
For each gearbox, record the information consistently.
A basic field sheet might look like this:
MeasurementExisting GearboxManufacturer__________Model__________Serial__________Ratio__________Output center height__________Longitudinal bolt spacing__________Transverse bolt spacing__________Mounting-hole diameter__________Base length__________Base width__________Output-shaft diameter__________Output-shaft extension__________Output key__________Input-shaft diameter__________Input-shaft extension__________Input key__________Overall length__________Overall width__________Overall height__________Coupling spacing__________Torque-arm dimensions__________Flange dimensions__________Hollow-shaft bore__________Notes__________
Add:
Units: IN / MM
at the top of the form.
Never leave units ambiguous.
Add a Simple Measurement Sketch
A rough sketch can make field measurements much easier to interpret later.
Draw the gearbox from:
side,
top and
shaft end.
Label dimensions:
A, B, C, D, E...
Then create a table:
DimensionMeasurementA — Center height10.00 in.B — Bolt spacing22.00 in.C — Bolt spacing14.00 in.D — Output shaft3.000 in.E — Shaft extension6.00 in.
The drawing does not need to be beautiful.
It needs to make the measurements unambiguous.
Photograph the Measuring Process
For important dimensions, take a photograph showing where the measurement was taken.
For example:
caliper on output shaft,
tape between mounting holes,
straightedge establishing mounting plane,
measurement to shaft centerline.
This can be extremely useful when an engineer or supplier reviews the information remotely.
They can see what you actually measured.
Do Not Round Critical Measurements
Avoid entries such as:
Shaft ≈ 3 in.
if the exact shaft dimension matters.
Instead record:
Shaft measured: 2.998 in.
or
Shaft measured: 74.98 mm
depending on the equipment and measurement accuracy.
For large overall dimensions, extreme precision may be unnecessary.
For:
shafts,
pilots,
bores,
keys and
fits,
precision can be critical.
Separate “Measured” From “Assumed”
A good replacement worksheet should distinguish between:
Verified From Manufacturer Drawing
High-confidence documented dimension.
Field Measured
Measured directly from equipment.
Estimated
Approximate measurement because access was limited.
Unknown
Not yet verified.
This prevents an estimate from accidentally becoming an “official” dimension after being copied through several spreadsheets.
Don't Forget the Mounting Plate
Sometimes the dimensions you see do not belong to the gearbox itself.
An old gearbox may already be installed on:
adapter plate,
fabricated base,
riser,
shim pack or
OEM mounting bracket.
Document these separately.
For example:
Gearbox center height: 9.00 in.
Adapter plate thickness: 1.00 in.
Installed shaft center height: 10.00 in.
That distinction can open additional replacement options.
A gearbox with a different native center height may still be installed successfully by redesigning the adapter.
Compare the Existing and Proposed Gearbox Side by Side
Once you have the replacement drawing, create a comparison table.
DimensionExistingReplacementDifferenceActionCenter height10.00 in.10.00 in.0NoneBolt spacing A22.00 in.23.00 in.+1.00AdapterBolt spacing B14.00 in.14.00 in.0NoneOutput shaft3.000 in.3.000 in.0NoneShaft extension6.00 in.5.25 in.-0.75Check couplingOverall length36.0 in.38.5 in.+2.5Check clearance
Now the plant knows what “replacement” actually means.
When Should You Stop Measuring and Call a Supplier?
You do not necessarily need to reverse engineer the entire gearbox before contacting a supplier.
If you have:
manufacturer,
model,
serial number,
ratio and
clear nameplate photograph,
start there.
A gearbox specialist may be able to identify the unit and locate drawings.
If the gearbox is obsolete or unidentified, additional measurements become increasingly important.
A good initial package includes:
nameplate,
full gearbox photos,
motor information,
application,
center height,
bolt pattern,
output shaft and
mounting arrangement.
From there, the supplier can identify which additional dimensions are needed.
Three Measurements Can Quickly Eliminate the Wrong Gearbox
Center height, bolt pattern, and shaft diameter do not prove that a gearbox is suitable.
But they can quickly reveal that one is not a direct physical replacement.
Consider:
Existing Gearbox
Center height: 10 in.
Bolt pattern: 22 × 14 in.
Output shaft: 3.000 in.
Proposed Gearbox
Center height: 12 in.
Bolt pattern: 25 × 16 in.
Output shaft: 3.500 in.
Even before reviewing every other specification, you know this is not a simple drop-in replacement.
That information can save time during the sourcing process.
But Dimensions Alone Are Not Enough
The opposite mistake is also possible.
Two gearboxes can have nearly identical dimensions and still be mechanically incompatible.
After confirming fit, verify:
ratio,
input speed,
output speed,
torque,
horsepower,
service factor,
thermal capacity,
overhung load,
thrust load,
rotation,
backstop,
lubrication and
application suitability.
See Why Matching the Gear Ratio Isn't Enough When Replacing a Gearbox for the performance side of replacement selection.
How This Differs From a Full Dimensional Review
This field guide focuses on how to obtain the measurements.
For a more complete discussion of all dimensions that may affect gearbox installation, see Gearbox Replacement Dimensions: The Measurements That Determine Whether a Reducer Will Fit.
That guide covers the broader dimensional review.
This article is the one to hand to the technician standing beside the machine with a caliper and tape measure.
Measurements Become Critical With Obsolete Gearboxes
If the original gearbox is no longer manufactured, dimensional information can become the foundation of the replacement project.
The supplier may need to determine whether a current gearbox can:
bolt to the existing base,
align with the driven equipment,
connect to the existing shaft or
be adapted economically.
Our guide How to Replace an Obsolete Gearbox When the Manufacturer No Longer Supports It explains the available options.
Sometimes the best replacement is not identical.
It is the gearbox that meets the application requirements while requiring the fewest practical modifications.
Add These Measurements to the Asset Register
Do not allow the measurements to disappear after the project.
Add them to the gearbox asset register.
For critical gearboxes, store:
dimensional drawing,
measurement sheet,
photographs,
approved replacement,
required adapter drawings and
coupling information.
Then the next gearbox failure begins with known information.
Measure Critical Gearboxes Before They Fail
If a plant has older or difficult-to-source gearboxes, there is no reason to wait for a breakdown.
Prioritize units that are:
production critical,
obsolete,
custom,
poorly documented,
long lead time or
missing a spare.
During a planned maintenance window:
isolate the equipment safely,
remove guards as required,
photograph the installation,
verify the nameplate,
measure center height,
measure the bolt pattern,
measure shaft dimensions,
document connections and
store the information.
A small amount of planned work can eliminate a great deal of emergency uncertainty.
The Field Checklist
Before leaving the machine, make sure you have:
Asset number
Manufacturer
Complete model
Serial number
Ratio
Nameplate photo
Full installation photos
Motor nameplate
Center height
Longitudinal bolt spacing
Transverse bolt spacing
Mounting-hole diameter
Output-shaft diameter
Output-shaft extension
Output keyway
Input-shaft information
Coupling information
Base dimensions
Overall envelope
Flange or hollow-shaft information where applicable
Torque-arm information where applicable
Accessory clearances
Units clearly recorded
Photos showing critical measurements
If you have those items, a supplier or engineer will have a much better starting point for evaluating replacement options.
Measure First, Order Second
A replacement gearbox should not be selected by appearance.
And it should not be selected by ratio alone.
For physical interchangeability, begin with the dimensions that establish the relationship between the gearbox and the machine:
Center height.
Bolt pattern.
Shaft diameter.
Then work outward to:
shaft extension,
keyway,
input arrangement,
coupling,
mounting base,
overall envelope and
accessories.
Combine those measurements with the gearbox's operating requirements.
That is how you move from:
“This reducer looks about right.”
to:
“We know exactly how this reducer will fit.”
Need Help Measuring or Identifying a Replacement Gearbox?
Industrial Gearbox Supply can help evaluate replacement options for standard, obsolete, and difficult-to-identify industrial gearboxes.
Start with:
clear nameplate photograph,
full gearbox photographs,
manufacturer,
model,
serial number,
ratio,
motor horsepower and RPM,
application,
center height,
bolt pattern and
output-shaft dimensions.
If an exact replacement is unavailable, additional measurements can be used to determine whether a current reducer can be adapted to the existing machine.
A few accurate measurements taken before ordering can prevent expensive modifications after the gearbox arrives.
Sources
OSHA — Control of Hazardous Energy (Lockout/Tagout)
General information on controlling hazardous energy during servicing and maintenance activities.
https://www.osha.gov/control-hazardous-energy
OSHA — 29 CFR 1910.147, The Control of Hazardous Energy
Federal general-industry requirements concerning hazardous-energy control during servicing and maintenance.
https://www.osha.gov/laws-regs/regulations/standardnumber/1910/1910.147
American Gear Manufacturers Association (AGMA)
Industry standards and technical resources relating to industrial gearing, enclosed gear drives, ratings, terminology, and applications.
https://www.agma.org/
SEW-EURODRIVE — Industrial Gear Units
Manufacturer resources covering industrial gearbox configurations, mounting arrangements, shaft options, accessories, and technical documentation.
https://www.sew-eurodrive.com/products/gear_units/industrial_gear_units/industrial_gear_units.html
NORD DRIVESYSTEMS — Industrial Gear Units
Manufacturer information covering industrial gear units, shaft and mounting configurations, dimensional options, and drive-system applications.
https://www.nord.com/en/products/industrial-gear-units/industrial-gear-units.jsp
Sumitomo Drive Technologies — Gearboxes and Gearmotors
Manufacturer resources covering industrial gearbox configurations, shafts, mounting arrangements, ratios, and application selection.
https://us.sumitomodrive.com/en-us/products