Western Railroad Discussion > Wheel diameter measurements?

Where are wheel diameters measured?

At the tread or on the outside edge of the flange?

TIA

From the witness mark on the side of the wheel.

Nudge

Could you be more specific, don't understand the term. Thanks!

The witness mark that spnudge is referring to is a groove in the side of the wheel tread. It can be seen in this photo:

http://www.nsdash9.com/images/NS3239bh23.jpg


Chris Toth
NSDash9.com

The witness groove is a known diameter but most wheels are measured with a wheel gauge that fits on the back of the wheel and to the center of the tread. Wheels are measured in 16ths down the back of the wheel. Gauges shown are wheel diameter with finger gauge for measuring flange thickness and a go, no go gauge for flange and wheel tread.

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The witness groove is extremely important when investigating an incident between the locomotive and something else. A 40" wheel has a groove exactly 36" in dia. The ACTUAL wheel dia. can be measured and can be used in computing the ACTUAL speed of the locomotive at the tome of the incident.

I think that, with the all the legal issues surrounding a RR vs. something else, TRUE speed can be critically important in resolving certain situations.

qman



Edited 1 time(s). Last edit at 11/03/11 18:51 by qman.

HOw do you determine the speed by just knowing the diameter of the wheel? Seems to me you have to know the rpm of the wheel to do that combined with the measurement of the wheel.

The older Pulse data recording system required the input of wheel diameter to calculate speed. The locomotives in recent times use radar to control wheel slip as well as determining true ground speed. No measurements are required on these engines.

The critical nature of wheel dia. was such that, when removing the 8 track tape on which the data was recorded, the person removing the tape was admonished to ascertain the dia. of the wheel to which the speed recorder was attached.

qman

Knowing the wheel diameter is very important on locomotives for the footage counter to be accurate, it must be calibrated to the wheel size. Most locomotives use both axle drive and radar for speed. The older SD70m's use axle drive on the speedometer in the console, but the speed recorder in the computer is radar.

When turning wheels on a lathe there are very close tolerances between wheels on each end of the same axle and on the same truck. This is sometimes checked with a steel tape applied to the tread at a set distance from the rim. I don't know the tolerances permitted. This is to control any tendency of the axle to steer due to different diameters.

In a shop, wheels are measured using a Wheel Circumference Gauge, or "Tape". The Tape has stand offs that hold it at a fixed distance from the flange, so each wheel is measured at the same point on the tread, which is the center line. Per the AAR Wheel Shop rules, new wheels mounted on the same axle must be the same "Tape Size", as marked on the wheel, and second hand wheels must be within One Tape Size, that is 1/8" of circumference...not a lot.

The witness groove on locomotive wheels is not commonly found on car wheels. It merely provides a reference point on the front of the rim to indicate where the back of the rim ends and the wheel plate begins. Wheel measurements in service are taken on the back of the rim using the Steel Wheel Gauge. Because of the equipment commonly present on a locomotive axle (sanders, flange lubricators, gear cases, etc.) it difficult to use the steel wheel gauge on the back of the rim under a locomotive, and that is where the witness groove comes in.



Edited 2 time(s). Last edit at 11/04/11 11:30 by SilverPeakRail.

Wheel diameters are various...either they are serviceable
or condemned...

Speaking of wheels, I am always amassed when hearing a "flat" wheel pounding along that they can hold up to the pounding.