5

Does anybody have a suggested "warm up" procedure for a small 1/4" drive torque wrench? I've searched the internet and found surprisingly little on this topic.

Here is how I've been doing it so far:

1) Set the torque wrench to a setting at the bottom of its range, ex. if the torque wrench range is 2 Nm to 20 Nm, set it to, say, 3 Nm (it should go without saying that the wrench should have been stored at the min value on the scale, 2 Nm in our example).

2) Find a bolt that is at least substantially more, for example at least 5 Nm, and gently / smoothly turn the wrench on that bolt until it clicks (without the bolt moving of course, if it moves something is seriously wrong, have your torque wrench recalibrated or use a different one). Alternatively you can put a bolt in a vise and click it on that.

3) Repeat perhaps 5 to 10 times, waiting a few seconds in-between each to allow the internal lube time to spread out.

Is this an acceptable warm up procedure for a small 1/4" drive torque wrench commonly used on bikes? Does anybody have a different suggested procedure?

  • 9
    IMHO 'Warming Up' is more about reminding the 'Gorilla with the spanner' what a 'click' feels like before he starts to torque real bolts and breaks something expensive. The Torque Wrench works just fine without warmup. Also I often do a couple of clicks on a suitable bolt to check the wrench is working correctly before heading over to my Carbon bike. – mattnz Nov 20 '16 at 7:27
  • 2
    Unless the wrench has not been used for years, there should be no need to "warm it up". (Except, perhaps, when working with something fragile, like a carbon bike.) – Daniel R Hicks Nov 20 '16 at 13:25
  • 8
    "preheat oven to 180 degrees for 20 minutes. Warm wrench at 200 degrees for 8 minutes. Use oven mitts to handle wrench. Reheat when wrench is cool enough to touch without sizzling sound from flesh". – Móż Nov 21 '16 at 1:16
  • 3
    @Móż sizzling sound from flesh reminds me of being a physics student learning scientific glass blowing. Once the piece of work has stopped glowing it's easy to pick it up with the hot end. Since it's taken several hours to make, one doesn't want to follow the urge to throw it down, hence sizzling sound from flesh! – andy256 Nov 21 '16 at 1:40
  • 1
    @andy256 similar with metalwork, the gap between "not glowing" and "cool enough to hold" can be exciting. Metal things are not always as solid as you might think - you can really screw up thin wall bicycle tubing by dropping it the wrong way. Or just char through leather gloves. Also, in case it's not obvious: the above is not good advice. Do not do that. – Móż Nov 21 '16 at 1:44
11

Some people claim that warming up a torque wrench necessary, but I've never seen it actually seen a manufacturer (Park Tool, Snap On, etc.) recommend a warm up procedure (*) so I don't see it as necessary.

I just use torque wrenches/keys directly. As Zinn notes (see for example, Appendix D of Zinn & the Art of Mountain Bike Maintenance 5e), you have a lot of things that can affect a torque reading:

  • Lubrication (or condition in general) of threads
  • Position of using the torque wrench, extenders, etc.
  • Choking up on the torque wrench (your hands will be more sensitive, and on many wrench designs it doesn't affect the reading)
  • Corrosion
  • Materials
  • Bolt turning vs starting from stuck (static vs kinetic friction)
  • Calibration (which can be thrown if you use it to loosen bolts or drop the wrench repeatedly or just through (ab)use)
  • Etc.

If you're working with carbon fiber or light weight materials, you have to be careful, but for most parts, its not the end of the world if you over/undertighten by a bit (though of course, its better to be at the right value). Having a sense of what the torques feel like is a good thing to have -- most people do just do these things by feel on normal grade parts. Using torque = arm length * force (assume force is applied perpendicular to the arm), you can get a rough sense of how much force you need. A liter of water has 1 kg mass, so its weight is about 10 Newtons. So, if you apply it at on an arm of about 20 cm (roughly the length of many small torque wrenches), you get a torque of 2 N-m.

(*) Life's full of these sort of things: For example, waiting for your car to warm up in winter before driving, rather than just driving gently after you turn it on.

  • Er ... is that spam? – andy256 Nov 20 '16 at 5:53
  • 6
    Being a fan of Click & Clack, I think I can infer that Batman's link is a gentle suggestion that warming up a torque wrench is the kind of practice that one often learns from one's father, which could prove to be completely bogus. – rclocher3 Nov 20 '16 at 6:16
  • 4
    @rclocher3 - yep. Car Talk was a US radio show where people would call in with their car (or some times other) questions to two mechanics in Boston who would try to diagnose the problem based on the calls (and succeed surprisingly often). One of the big problems people had was that their fathers or whatever told them to do some procedure, which may have been true 20-30 years ago or never at all and they stuck with it. Warming a torque wrench seems like one, since I've never seen a manufacturer recommend it prior to use but it seems to be passed down through folklore. – Batman Nov 20 '16 at 15:23
4

Torque wrenches don't warm up, although they should be at a normal room temperature, extreme cold could change the spring rates and such. They don't need any break-in either, (good brands here) each wrench is tested at several torques before leaving the factory. While it may be nice to have the wrench tested every few years just for a calm mind, most stay very close to tolerance. Our shop would torque about a quarter million fasteners per year with a clicker type, and the wrenches were tested every year, none were ever significantly off of spec. Re-torquing a bolt is not a torque test and really tells you nothing, calibration testing needs a particular device and I won't go into all that here. Lastly the torque wrench is rarely to never the weak link in the fastener installation. Even if the bike factory did actual testing(most don't they just use charts with fudge factors) you still have the exact type of lube, the placement of lube(just threads, under the head, or both), metallurgy of the tidbits, and operator technique. All of which dwarf the uncertainty in the actual torque wrench.(again talking of good brands) Just selection of lube can cause a 50% change in axial load on the fastener, exact same torque setting.

  • I've heard the variation to be closer to +- 15-25%. – Batman Nov 21 '16 at 18:01
  • 50% is the far end to far end of tolerance stacking. +-15% is the accuracy when reasonably controlling the variables. Using the specified material finish(chrome, zinc, oxide, bare, etc) and with lubrication that is in the ball park of what was used to develop the test, including both type(dry, high pressure grease, plain oil, etc) and application(threads only, under head/nut only, both). – Max Power Jan 5 '17 at 22:52
  • For better accuracy than that, the exact lubricant formulation, surface condition, temperature, technique and parts supply that was used in developing the torque spec must be used. Even then variation from screw to screw will still throw it off a few percent. Fasteners needing better than 10% generally use light torque plus angle of rotation. For very critical parts, the stud or bolt is preloaded directly by a hydraulic device that tensions the fastener without twisting and the nut is screwed down hand tight. (eg heads on giant marine diesels) Goal is axial preload, not torsional friction. – Max Power Jan 5 '17 at 22:58

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.