Torque Testing / Gear Operator Ratio Question 1

[BigSerg]

Hello All,

I have a 12" 900# Slab Gate Valve that I intend to perform a differential pressure torque test on. During the torque test, the valve will be equipped with a gear operator and pressurized on one side to the Maximum Operating Pressure (2160 psig). I have rigged up a fitting to be able to mate a digital torque wrench to the stem of the gear operator. However, I need to know the torque required to open the valve under pressure at the stem of the valve (without the gear operator). How could I determine the gear ratio of the gear operator since I will need that value to calculate the breakaway torque at the stem? Would the number of gear operator handwheel revolutions required to bring the valve from a fully closed to fully open position be my gear ratio? Sorry if this is a rather elementary question. Any help is appreciated. Thanks in advance.

 

[gerhardl]

To comment:

a) Torque measured at input stem of gear operator. In case: breakaway torque at valve stem / input torque at gear operator stem = gear ratio.

b) Gear ratio is normally labeled at the gear or given in data sheets from the supplier. Simplified: Controlled and measured as ratio between turns of output/turns of input (For two wheels without cogs circumference of output wheel/circumference of input wheel, for cogwheels number of teeth). For other types of wheels calculators exists. Note: Slider-crank gearboxes will reinforce output and give variable reinforced (by level arm) output: search data sheets.

http://www.ehow.com/how_5993234_figure-gear-ratio.html

c) Do not forget that torque is force. Effect (work) is force x time.

d) Check that valve is allowed by factory to be tested at the given maximum operating pressure. Some slab valves have limits to operation and could possibly be limited to allowed opening and closing at given (even operational) pressures and/or max pressures. Limits will vary from producer to producer.

e) Internal friction will add slightly to loosening torque. Loosening torque will however vary very much over lifetime and largly possibel by repeated tests. Check that valve is serviced and properly greased, if applicabel. At variabel circumstances safety factor of applicabel max torque could be difficult to pinpoint.

Hope I did this correctly. Good luck!

 

[cloa]

Work is Force times distance. If you push a wrench and it doesn't move then you do no work except the tiny bit on moving its molecules by heat and inside your body.

http://www.eng-tips.com/threadminder.cfm?pid=1529

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[LittleInch]

Why do you need to know? I can't imagine a 12" #900 gate valve without some sort of geared actuator as a wheel will just be too difficult to open manually so what is the point?

Your gear ratio is the number of turns of your gear operator to a single turn of the main valve gear which is driving the slab open presumably via a screw gear in the middle of the main valve gear. You therefore need to be able to see the main gear wheel or have it known how many revolutions of the main gear wheel are required to move the valve from fully open to fully closed. Then it becomes number of gear operator rotations to go from FO to FC divded by the number of main gear revolutions to go from FO to FC. ALternatively ask the gear operator vendor.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[BigSerg]

@ LittleInch...You got it right on the money sir...Thank you very much.

Now, if my gear ratio is 4.33:1 and I'm able to measure the amount of torque required to turn the handwheel on the gear, what formula will I use to calculate the break torque at the stem? Thanks.

 

[LittleInch]

I might get shouted down here but for a static breakaway value, I can't see beyond multiply by 4.33. Running torque you will loose some efficiency in the gears, but static or near static torque seems fairly obvious.

I must admit I'm still a bit worried by this breakaway torque issue. Normally this is a ball valve issue more than a gate valve one where all you really need to know is what is the value on the end of you actuator spindle and you are measuring that...

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[al1979]

well you can't measure torque without gearbox or 'controlled' torque testing hydraulic machine.

Follow the steps

1) Remove handwheel of gearbox
2) Make a coupler suitable to gearbox shaft on which handwheel was fitted and other side of coupler according to TORQUE WRENCH.
3) As per all known international standards and customer specifications, input force at handwheel shall not exceed 360 N. So use corresponding torque wrench.
4) Now set torque wrench value to minimum and try to open the valve under pressure. Increase the limit in steps till you open the valve.
5) Now, Valve torque under presssure = input torque of torque wrench X Mechanical advantage (MA) of gear box which probabely you will find it on name plate of gear box or may be in catalogue

Cheers !!!!!!

 

[siavash1111]

Torque normally defined as Thrust * Stem Factor (stem factor based on the type of ACME thread being used on stem of the valve )
you will need to calculate thrust at max MOP for first step then you can convert that to torque with some formula
i have attached some catalog for your information hope this help
ML040280163.pdf page 42
cv-422.pdf page 4

 

[bao2ye]

Torque= force X distance ( the vertical distance between rotational point to the force vector).