Neometrix / Rail Test Equipment / Speedometer Test Bench / NMX-STB-71

NMX-STB-71 · ENGINEERED-TO-ORDER CLASS — SPECIFY · REFERENCE · DRIVE · BUILD · TEST · PROVE

Speedometer Test Bench. True only to the wheel diameter.

Name: Speedometer Test Bench
Brand: Neometrix
SKU: NMX-STB-71

A speedometer on a locomotive or a motor coach does not measure speed. It counts the turns of an axle and multiplies them by the size of the wheel. So its reading is only as true as the wheel diameter set in it.

A bench for it therefore has to do three things: drive the instrument at known speeds, compare it with a reference that is better than it, and record the wheel diameter it was set to.

Discuss a speedometer bench Capability note (PDF)

A single speedometer test bench on a plain white studio background, seen from the side: a long flat grey painted steel base plate carrying, in one straight line on a common shaft axis, a grey finned electric motor on a steel foot, a bright bellows coupling, a cylindrical bright aluminium pulse generator held in a machined steel clamp block, and a smaller cylindrical encoder at the far end with a looped black cable, and no people

Fig · 01 — A speedometer test bench: a motor, a precision coupling and a pulse generator held in a clamp block on one axis, with a reference encoder at the far end — illustrative render.

The reading

axle turns and wheel sizethat is all the instrument knows

The reference

better than the instrumentwith a certificate the customer names

The speeds

several, rising and fallingacross the whole range

The record

diameter setting keptwith every instrument tested

Status

engineered to orderno speedometer bench claimed

ISO 9001ISO 14001ISO/IEC 17025 reference standard

Overview

Why the whole job is the wheel.

Because a speedometer reads the wheel, and the bench has to make that reading honest.

A single axle-end pulse generator on a plain white studio background: a cylindrical bright aluminium housing with a round mounting flange carrying four round bolt holes, a short keyed steel input shaft at the front, and a short black sheathed cable leading to a round multi-pin connector, and no people

Fig · 02 — An axle-end pulse generator: a cylindrical housing with a mounting flange, a keyed input shaft and a cable to a round connector — illustrative render.

A speedometer is only as true as its wheel diameter.

What the bench is for

It tests the speed indicators and recorders of locomotives and motor coaches away from the train. It turns a real pulse generator at set speeds, or feeds the instrument the pulses a chosen speed would make, and compares what the instrument says with a reference. It suits a motor-driven bench with its pulse generator, a pulse simulator for the indicator alone, and a combined bench with both.

Why a speedometer is only as true as its wheel diameter

A speedometer driven from an axle does not measure speed. It counts the turns of the axle and multiplies them by the size of the wheel. If the diameter set in the instrument is not the real one, the reading is wrong by the same proportion.

A worn wheel travels less far per turn than the instrument assumes, so it reads fast. The setting is entered by hand or by a switch, and it has to follow every change in the wheel.

Why the bench proves the instrument and not the wheel

The bench supplies a known turning rate or a known pulse rate and compares the reading with a reference. That proves the instrument for the diameter it was told. The wheel itself is measured at the wheel, so the diameter setting is recorded with every test and checked again after the wheels are turned.

Why the reference comes first

A reference has to be better than the instrument it checks, and its certificate has to come from a laboratory the customer names. A bench is only as good as the thing it compares against.

Why several speeds, rising and falling

An instrument is tried at several speeds across its range, both rising and falling, because a reading can lag or stick in one direction. Any over-speed contact is tried at its set point.

Why the coupling and the jig matter

A pulse generator turned through a coupling that is out of line or loose gives an unsteady pulse train. The jig therefore holds each sensor on the same axis as the motor, and the coupling is chosen to carry no side load.

The method

Specify, reference, drive, build, test, and prove.

Six steps in a fixed order. Choosing the reference before the motor, and keeping the wheel diameter in the record, are the two a quick order skips.

1 · Specify

The instruments and pulse generators to be tested, and how they are mounted, are stated by the customer.

2 · Reference

The reference is chosen to be better than the instrument, with a certificate from a laboratory the customer names.

3 · Drive

The motor, drive, coupling and jigs are chosen to turn each sensor smoothly on one axis, and a frequency source is added for the simulated pulses.

4 · Build

The bench, the jigs and the console are built as one installation, and the over-speed contacts and recorder output are wired to it.

5 · Test

The bench is proved against a reference and a known pulse generator before any customer instrument is put on it.

6 · Prove

A record is kept for every instrument, with its speeds, its readings and its diameter setting, and handed over with the bench.

Which way to drive the instrument

Method	How it works	What to watch
A motor turning a real pulse generator	a motor turns a genuine pulse generator or tachogenerator through a precision coupling, and the instrument is read at set speeds	it tests the sensor as well as the indicator, but each type needs its own jig, and a coupling out of line spoils the pulses
A simulated pulse train	a frequency source gives the indicator the pulses that a chosen speed and a stated wheel diameter would make	it is quick across many speeds, but it tests the indicator alone, and the frequency source has to be as traceable as any other reference
A test run on a track	the instrument is fitted to a locomotive or a coach and compared with a reference over a measured length	it tests the whole system, wheel included, but it is slow, costly and hard to repeat, which is why a bench comes first

The step people underrate is the record of the wheel diameter. A bench that proves an instrument and forgets the diameter has proved only half of the reading.

Work content

What the bench contains, element by element.

Read it as a checklist: a bench missing a row will pass an instrument on paper and not on the train.

A single railway wheelset on a plain white studio background: two identical solid steel wheels with bright machined treads and flanges and grey painted centres on one grey axle, with a round steel end cover and a small cylindrical aluminium sensor housing bolted to one axle end, and no people

Fig · 03 — A wheelset with a sensor housing on one axle end: the instrument counts the turns of the axle and multiplies them by the size of the wheel — illustrative render.

Element	What it does	What matters
Bench base and frame	carry everything on one axis	heavy, flat and stiff, so that the motor, the coupling and the sensor stay in line however the bench is loaded
Motor and drive	turn the sensor	a variable-speed drive that holds a steady speed at every point, rising and falling, with no hunting
Precision coupling	joins motor to sensor	a coupling that carries the turning and no side load, so that the pulse train is steady
Jigs and clamp blocks	hold each sensor	machined to the mountings of the sensors the customer names, and quick to change from one type to another
Reference encoder and counter	measure the true speed	better than the instrument they check, with a certificate from a laboratory the customer names
Frequency source	stands in for the pulse generator	gives the pulses that a chosen speed and a stated wheel diameter would make, and is traceable like the reference
Test console and record	run the test plan	the speed points, the readings and the wheel diameter setting, kept for every instrument
Over-speed and recorder interface	tries the outputs	any over-speed contact tried at its set point, and the recorder output read against the reference
Procedure	fixes the sequence	specify, reference, drive, build, test and prove, the same for every instrument
Testing & documentation	prove the bench first	the bench itself checked against a reference before any instrument, and a record the customer can inspect

The element that decides whether the bench can be trusted is the reference, because a bench cannot be truer than the thing it compares against, and the record of the wheel diameter has to travel with every reading.

Full specification — expand

System	A heavy bench base carrying a variable-speed motor and drive; a precision bellows coupling; machined jigs and clamp blocks that hold different pulse generators and tachogenerators on one common axis; a reference encoder and counter that are better than the instrument they check; a frequency source that can stand in for the pulse generator; a test console with the test plan, the speed points and a record for every instrument including the wheel diameter setting used; the interface for any over-speed contacts and the recorder output; a written test plan; and testing and documentation before handover
The One Idea	A speedometer is only as true as its wheel diameter.
Why A Speedometer Is Only As True As Its Wheel Diameter	A speedometer driven from an axle does not measure speed: it counts the turns of the axle and multiplies them by the size of the wheel, so if the diameter set in the instrument is not the real one the reading is wrong by the same proportion, and a worn wheel travels less far per turn than the instrument assumes and so reads fast
Why The Bench Proves The Instrument And Not The Wheel	The bench supplies a known turning rate or a known pulse rate and compares the reading with a reference, which proves the instrument for the diameter it was told, while the wheel itself is measured at the wheel, so the diameter setting is recorded with every test and checked again after the wheels are turned
Why The Reference Comes First	A reference has to be better than the instrument it checks, and its certificate has to come from a laboratory the customer names, so a bench is only as good as the thing it compares against
Why Several Speeds Rising And Falling	An instrument is tried at several speeds across its range, both rising and falling, because a reading can lag or stick in one direction, and any over-speed contact is tried at its set point
Why The Coupling And The Jig Matter	A pulse generator turned through a coupling that is out of line or loose gives an unsteady pulse train, so the jig holds each sensor on the same axis as the motor and the coupling is chosen to carry no side load
Standards	ISO/IEC 17025 is the public standard for the competence of calibration laboratories, and the reference of a bench is traced through a laboratory of that kind that the customer names. This page prints none of the tolerances, speeds, diameters or wear figures of any specification, and no edition year: those come from the specification the customer names and from the instrument. Approval of a bench or of an instrument rests with the customer and the authority they name; none is claimed here
Configurations	A motor-driven bench for speed indicators and recorders with their pulse generators, a pulse simulator bench for the indicator alone and a combined bench with both
Scope Boundary	This page is the speedometer test bench. It is not a brake test rig, which proves air brakes at coach and train level (see single wagon, single coach and rake test rigs), not a bogie rig (see rail bogie test rig and turntable) and not a pressure calibration bench, which compares a gauge with a reference pressure (see pressure calibration bench). A wheel profile gauge and a wheel lathe are different equipment again. No certification is claimed here
Status	Neometrix engineers speedometer test benches to order, and no delivered speedometer test bench is claimed.

Configurations

One method, three ways to build the bench.

The method, the reference and the record of the wheel diameter are shared. What changes is how the instrument is driven.

Motor-driven bench

Motor, coupling and jigs

A motor that turns a real pulse generator or tachogenerator through a precision coupling, for the speed indicators and recorders that are overhauled in the shed.

Pulse simulator bench

Frequency source and counter

A bench that gives the indicator the pulses a chosen speed and a stated wheel diameter would make, for a quick check of the indicator alone.

Combined bench

Both, with one record

A bench with the motor-driven line and the simulator side by side, and one record for every instrument tested on either.

And the part that is not equipment at all, yet decides all three: the test plan. It fixes the speed points, the direction of approach, the way the reference is checked, and what is recorded, the wheel diameter setting included. It is written from the instruments, not from the motor.

Where it is used

Wherever a speed reading is trusted, and the wheels keep changing.

The common thread is a speed reading that people act on, and wheels whose diameter does not stay the same.

Locomotive and coach sheds

Where speed indicators and recorders are overhauled and have to leave the shed trustworthy.

Multiple-unit depots

Where many identical instruments are checked in turn and the record has to be the same for each.

Instrument and calibration shops

Where speed instruments are compared with a reference and a certificate is issued.

Wheel shops

Where turning a wheel changes its diameter, and the setting in the instrument has to change with it.

Watch

See it in motion.

A short walkthrough video of the speedometer test bench.

FAQ

Common questions.

Longer answers, for readers who want the reasoning.

Q · 01 What does a speedometer test bench actually do?

It checks a speed indicator or recorder away from the train. A motor turns a real pulse generator at set speeds, or a frequency source gives the instrument the pulses a chosen speed would make, and the reading is compared with a reference. Any over-speed contact is tried at its set point and the recorder output is read. It does not measure a wheel, it does not set the diameter on a train, and it does not replace a test run.

Q · 02 Why is a speedometer only as true as its wheel diameter?

Because it does not measure speed directly. Driven from an axle, it counts turns and multiplies them by the size of the wheel. If the diameter set in it is wrong, every reading is wrong by the same proportion. A wheel wears and is turned back to shape, so its diameter falls over its life, and a worn wheel travels less far per turn than the instrument assumes. The reading then runs fast. The setting is entered by hand or by a switch, so it stays true only if someone changes it when the wheels change.

Q · 03 Why does the bench prove the instrument and not the wheel?

Because it never sees the wheel. It supplies a known turning rate or a known pulse rate, so it can prove the instrument for the diameter it was told. The real wheel is measured at the wheel, with its own gauge, and that diameter has to be set in the instrument. The bench therefore records the diameter setting with every test, so that the record shows what the instrument was told. After the wheels are turned, the instrument is set again and tried again.

Q · 04 Why is the reference chosen first?

Because a bench cannot be truer than the thing it compares against. The reference encoder and counter have to be better than the instrument, and their certificate has to come from a laboratory the customer names. Everything else is chosen to suit the reference. The motor must hold a steady speed so that the reference can be read, and the coupling must carry the turning without side load. A bench built around a motor and given a reference afterwards tends to prove less than it appears to.

Q · 05 Is this the same as the brake test rigs, the bogie rig or the pressure calibration bench?

No, and the differences are worth stating precisely. The single wagon, single coach and rake test rigs page covers air-brake testing at coach and train level. The rail bogie test rig and turntable page covers the equipment that proves a bogie. The pressure calibration bench page covers comparing a gauge with a reference pressure. This page is the speedometer test bench. A wheel profile gauge and a wheel lathe are different equipment again. It does not replace any of them, and it does not claim what they claim a second time.

Q · 06 Has Neometrix built one of these?

We would rather answer this plainly than let a page imply otherwise. Neometrix engineers speedometer test benches to order, and no delivered speedometer test bench is claimed. What stands behind the offer is adjacent: Neometrix engineers railway air-brake test rigs (see the single wagon, single coach and rake test rigs page), a rail bogie test rig (see the rail bogie test rig and turntable page) and a bench that compares an instrument with a reference (see the pressure calibration bench page). The motors, drives, encoders, frequency sources, counters and pulse generators are catalogue items, bought in, not invented. What Neometrix engineers is the bench around them: the frame and jigs, the coupling, the console and the record, the procedure and the test plan. So the honest position is that the bench is engineered to order, the neighbouring disciplines are in the building, and the first bench of this exact kind will be built around the customer's own instruments rather than taken off a shelf.

Q · 07 Which standard applies, and what can the bench not do?

ISO/IEC 17025 is the public standard for the competence of calibration laboratories. The reference of a bench is traced through a laboratory of that kind, which the customer names. This page prints none of the tolerances, speeds, diameters or wear figures of any specification, and no edition year: those come from the specification the customer names and from the instrument. The bench checks an instrument and keeps a record. It does not certify an instrument or a train: this page claims no certification or accreditation. No speed, diameter, tolerance or quantity figure is printed here, and approval for use rests with the customer and the authority they name; none is claimed here.

Q · 08 What do you need from us to quote?

Six things, and most of them describe your instruments rather than the bench. First, the instruments: the indicators and recorders to be tested, and how many types there are. Second, the sensors: the pulse generators or tachogenerators they use, and how they are mounted. Third, the range: the speeds the instruments must cover, and any over-speed contacts. Fourth, the reference: the certificate and the laboratory you trust. Fifth, the throughput: how many instruments are tested in a day. Sixth, the record: what you want kept with each instrument. From that we come back with a bench concept, a written test plan you can check, and a budgetary price.

The neighbouring rail and calibration pages, and how they differ from this one.

Three neighbours: one proves brakes, one proves a bogie, one compares a gauge with a reference.

↗ Brakes at train level

Single Wagon, Single Coach & Rake Test Rigs

Air-brake test rigs at coach and train level - a brake test, where this page checks a speed instrument.

↗ The bogie

Rail Bogie Test Rig & Turntable

Rigs that prove a bogie before it goes back under a coach - a test of the structure, where this page tests an instrument on the axle.

↗ Against a reference

Pressure Calibration Bench

A gauge compared with a reference pressure built from mass, area and gravity - pressure against a reference, where this page compares speed against one.

Browse all Neometrix product lines.

Get a quotation

Tell us the instruments, the sensors,
and the reference you trust.

The projects desk replies within two working days with a bench concept, a written test plan you can check, and a budgetary quotation. Write to [email protected] or use the form.

Enquire — speedometer bench Capability sheet (PDF) +91 7777 876 876

ISO 9001 / 14001 ENGINEERED-TO-ORDER CLASS — SPEEDOMETER TEST BENCH SPECIFY · REFERENCE · DRIVE · BUILD · TEST · PROVE — A SPEEDOMETER IS ONLY AS TRUE AS ITS WHEEL DIAMETER ENGINEERED IN NOIDA · INDIA