Why does force equal weight times g with no conversion?

Because pounds-force and pounds-mass are defined so that one pound of mass weighs one pound-force under one g. Accelerating 185 lb at 12 g therefore needs 2,220 lbf exactly, with no factor to remember. It is one of the few places the imperial system is genuinely more convenient than SI, where the same calculation carries a factor of 9.81.

Why does the armature matter so much?

Because it moves on every test and it is heavy. A mid-size electrodynamic shaker's armature runs from tens to well over a hundred pounds, and a head expander or slip table can double that before the specimen arrives. On a light specimen the shaker spends most of its force driving itself, which is why a 4,500 lbf machine can struggle with a 65 lb unit at a level the arithmetic on the specimen alone suggests is trivial.

My test fails the force check. What are the options?

In order of cost: reduce the fixture mass, which is usually the largest recoverable term and often achievable with a ribbed magnesium design in place of a solid aluminium plate; reduce the level if the specification permits, remembering force scales linearly with it; test a lighter representative article if the programme allows; or move to a larger shaker. Running the shaker over its rating is not on the list. It does not produce a valid test and it damages the machine.

What is a safe utilization to plan at?

Around 85%. The rating assumes an ideal rigid fixture and clean control, and real tests have neither. Fixture resonances inside the test band cause the controller to demand extra force to hold the profile, specifications get revised upward more often than downward, and fixtures come back from the shop heavier than they were drawn. The margin absorbs all three.

Does passing the force check mean the test will run?

No. Force is one of three limits. Displacement binds at low frequency, where reaching a given acceleration needs a stroke the shaker may not have, and velocity binds in the middle of the range. A 5 Hz test at modest g can exceed a shaker's stroke while using a fraction of its force. Check all three against the data sheet; this page covers the one that most often decides a specimen-mass question.

Why not keep the earlier version's booked-hours utilization?

Because it was a calendar metric that told you the shaker was busy and never whether it was capable, and a shaker that is idle all week is still no use for a test it cannot physically drive. Scheduling matters, but it is generic, every resource in the plant has a booked-hours ratio, while force capability is specific to this machine and is the question that stops test programmes. Both are honest; only one is worth a page of its own.

Acoustic, Noise, Vibration & NVH Products calculator

Shaker Table Force Utilization Calculator

Check whether a shaker can physically run a test before it is scheduled. Enter the shaker's force rating, the armature and fixture mass, the specimen mass and the required acceleration. The calculator returns the force the test demands, how much of the rating that uses, the highest acceleration reachable with this mass, and the largest specimen that could be tested at the required level. A shaker booked at 50% of its calendar is still useless for a test whose force demand exceeds its rating, and that is the constraint a utilization percentage never shows.

What this calculator does

Check whether a shaker can physically run a test: the force the moving mass demands at the required acceleration, against the shaker's rating.
Use it for checking a test profile against an existing shaker before quoting the work, scoping what specimen sizes a lab can accept at a given level, justifying a lighter fixture with the test capability it releases, assessing what a specification revision from 10 g to 15 g does to capability, comparing an in-house shaker against a subcontract lab's rating.
Check whether a shaker can physically run a test: the force the moving mass demands at the required acceleration, against the shaker's rating.

Formula used

Total moving mass = armature and fixture + specimen
Force required (lbf) = total moving mass (lb) × required acceleration (g)
Force utilization = force required ÷ shaker force rating × 100
Maximum acceleration = shaker force rating ÷ total moving mass
Largest specimen at the required level = rating ÷ required acceleration − armature and fixture

Inputs explained

Shaker force rating: Sine force rating from the shaker data sheet. Random and shock ratings are lower: use the one matching the test.
Armature and fixture mass: Moving element of the shaker plus the head expander, slip table or fixture. Paid on every test whether the specimen weighs anything or not, and the term most often left out.
Specimen mass: The unit under test, including any cabling or instrumentation moving with it.
Required acceleration: Peak acceleration the test profile calls for. For random vibration use the peak the controller must deliver, not the RMS.

How to use the result

Best suited to checking a test profile against an existing shaker before quoting the work, scoping what specimen sizes a lab can accept at a given level, justifying a lighter fixture with the test capability it releases, assessing what a specification revision from 10 g to 15 g does to capability, comparing an in-house shaker against a subcontract lab's rating.
Checks force only. Displacement and velocity limits bind at low frequencies and can stop a test the force check passes comfortably. Assumes a rigid fixture. A fixture with a resonance inside the test band amplifies locally, and the controller then demands far more force than this calculation predicts. Says nothing about overturning moment, which limits tall or offset specimens long before force does. Ignores duty cycle and cooling. A shaker can make its rated force and still not sustain it for a long random run. Not a schedule. It answers whether the test is possible, not when the shaker is free.

Common questions

Why does force equal weight times g with no conversion? Because pounds-force and pounds-mass are defined so that one pound of mass weighs one pound-force under one g. Accelerating 185 lb at 12 g therefore needs 2,220 lbf exactly, with no factor to remember. It is one of the few places the imperial system is genuinely more convenient than SI, where the same calculation carries a factor of 9.81.
Why does the armature matter so much? Because it moves on every test and it is heavy. A mid-size electrodynamic shaker's armature runs from tens to well over a hundred pounds, and a head expander or slip table can double that before the specimen arrives. On a light specimen the shaker spends most of its force driving itself, which is why a 4,500 lbf machine can struggle with a 65 lb unit at a level the arithmetic on the specimen alone suggests is trivial.
My test fails the force check. What are the options? In order of cost: reduce the fixture mass, which is usually the largest recoverable term and often achievable with a ribbed magnesium design in place of a solid aluminium plate; reduce the level if the specification permits, remembering force scales linearly with it; test a lighter representative article if the programme allows; or move to a larger shaker. Running the shaker over its rating is not on the list. It does not produce a valid test and it damages the machine.
What is a safe utilization to plan at? Around 85%. The rating assumes an ideal rigid fixture and clean control, and real tests have neither. Fixture resonances inside the test band cause the controller to demand extra force to hold the profile, specifications get revised upward more often than downward, and fixtures come back from the shop heavier than they were drawn. The margin absorbs all three.
Does passing the force check mean the test will run? No. Force is one of three limits. Displacement binds at low frequency, where reaching a given acceleration needs a stroke the shaker may not have, and velocity binds in the middle of the range. A 5 Hz test at modest g can exceed a shaker's stroke while using a fraction of its force. Check all three against the data sheet; this page covers the one that most often decides a specimen-mass question.

Last reviewed 2026-08-25.