Torque Wrenches: How Do They Work? Info Requested

[infamous_Q]

Hey, I've been looking around, and can't seem to find much. how do torque wrenches work? Regular ones, electrical/hydraulic ones, and the one's that check torques (either mechanical or electrical). Any info that can be given would be great...

actually, another quick question on that as well, at the factory I'm working at i have to do torque audits (use big wrenches to check torques on different parts to see if they're up to spec), do they have an electrical version of this to use?

Thanks a lot guys!

 

[Danger]

I only know about the simplest one, with a scale and pointer. The pointer is connected only at the business end, and so isn't subjected to the bending force that the handle is. It therefore deflects at an angle proportional to the torque (actually, it doesn't deflect; the handle does). The other end is suspended over a scale that converts that deflection to a ft/lb reading.

 

[Gokul43201]

In addition to the pointer type wrneches, described by Danger above, the only other kind I'm familiar with are the clicking ones where you set the limiting torque by rotating a circular scale on the wrench handle. These also use a simple mechanism.

Inside the shaft resides a spring which is fixed at the end nearer the handle. The other (free) end of the spring holds a ball pressed into a nearly hemispherical cavity at the base of the wrench head. Setting the torque by rotating the handle simply moves the fixed end of the spring closer to the head. This compresses the spring and hence linearly increases the force with with the ball at the other end presses against the head. The greater this pressing force, the greater is the torque it takes to rotate the head - by forcing the ball to pop out of the cavity.

I wish I had a picture to explain that.

Edit : Here's a dirty one, I just drew.

 

[Danger]

Cool! I never thought of it being that simple.

 

[FredGarvin]

The OP is referring to the wrenches used in a production environment. They are either electrically or pneumatically actuated. They are quite slick. I have seen them but I haven't looked into how they work or what the torque feedback mechanism is.

 

[NateTG]

One easy way to have a torque setting in a pneumatic wrench would be to simply regulate the pressure of the air supply either by means of a regulator or a bleed valve.

Since it's possible to control the stall torque of a motor by changing the supply voltage. I'd expect that's one method used for electrical torque wrenches. Alternatively, it's quite easy to have a torque sensor somewhere in the drive mechanism.

 

[Averagesupernova]

Most production level torque wrenches that are electric are either break away or like the cordless drills that have keyless chucks.

 

[FredGarvin]

A spring-pawl arrangement would be the most obvious form with the spring load being the adjustable variable. At the specified torque, the pawl would pop out of its recess and the torque delivered disappears.

I have a hard time believing that a pneumatic wrench could control torque with the use of a regulator. I'm not saying that's not how they do it, but the torque specs are usually pretty tight.

 

[aussiemecheng]

i have used various types of torque wrenches over the years, from the most basic to hydraulic and pneumatic, the basic mechanical torsion arm wrenches are the most reliable in as far as set torque compared to actual torque attained, these still require calibration at set intervals and adjustment to ensure accuracy within manufacturers specs or relevant standards body,,,,,,quality also affects accuracy, you get what you pay for with these. one limitation with these though is that you can only apply so much torque with the suckers, depends on the user, i have seen hand torque wrenches a metre and a half long and almost 50cm in dia on the handgrip,,,,mongrel buggers to use. You can however use a torque multiplier attachment with wrenches over 1/2" drive size to increase output torque or reduce operator strain,,,,these come with conversion tables to ensure torque multiplication is accurate.

The most operator friendly and accurate i found by far was the pneumatic type, provided your compressors load/unload cycle was correctly set to ensure minimum supply pressure drop during the unloaded stage.
The torque for these wrenches is set using an accurate high flow regulator, normally a balanced dual diaphragm type. Upon intial setup of this tool for the purchaser, a torque gauge is used to assess the available torque at different pressures as set at the regulator, normally in 10psi increments, using the purchasers air system at its nominal pressure.
A reference chart can then be produced to allow the tool operator to correctly set the tool to the required torque. Most of this type of tool are used for heavy torque application, i have personally used them upto ranges of 2500NM, 1850 FtLbs or so, mostly on heated and lubricated cap head bolts on polymer extrusion dies, but they can go upto 100,00NM+, ok as to how torque is generated, i will refer to a particular tool I am aware of for this, its powered by an airmotor coupled to a seven stage planetary reduction gearbox, this tool set at 50psi, had a torque output of 1500NM, with a supply pressure of 100psi, pretty impressive, the gear reduction results in a very slow output speed, additionally the tool is anchored by a reaction arm which allows the tool to apply full torque without spinning round in circles, and ripping your arms off.
Man that was long winded,
hope it explains ,,,,,um,,,,,something.

 

[Danger]

Very helpful. Thanks, Aussie.

 

[Bystander]

(snip)I have a hard time believing that a pneumatic wrench could control torque with the use of a regulator. I'm not saying that's not how they do it, but the torque specs are usually pretty tight.

Torque specs are "tight" because torque is a lousy way to set tension in fasteners, plus or minus 30% is about the best you can expect. If you've got a "tension critical" application, you go with load washers, acoustic measurement of bolt length, natural frequency of the bolt under the design load, or other techniques that are more closely related to actual fastener tension than is torque.

 

[brewnog]

Another way of torquing bolts in industry is to use a special solid socket extension in conjunction with a pneumatic impact air wrench. Once the desired torque is reached, the extension deforms. Because it's an impact wrench, it won't carry on twisting, and won't allow the bolt to be tightened any further.

I've seen these most commonly used for tightening things like truck wheels, but for critical applications (cylinder head bolts, for instance) a torque is set by traditional methods, and then the bolts are turned through a given angle. This is far more accurate than a torque setting alone.

 

[Bystander]

https://www.physicsforums.com/showthread.php?t=124528&highlight=torque+to+yield

 

[Shawnzyoo]

for a laboratory I had to calibrate a torque sensor that was based upon a strain gage.

http://www.omega.com/pptst/TQ103.html

Pricey but highly accurate and easy to use

You would input a known voltage and read the output voltage. The voltage would change dependent upon the torque loading.

Just another type that no one had mentioned yet.

 

[Danger]

Very cool, Shawn. I've never heard of that before. A little outside of my price range...