Torque wrench tolerance

[SIVAllan]

I haven't used a torque wrench in a while, but it was a manual wrench
and my Tiger engine survived to my great surprise.

I thought I got the various torques done right, but I would think that the wrench plus my entry skill level = unknown error.

Now I see torque wrenches offered by retailers that are settable, and make
a noise when it's time to stop, but the stated accuracy can be +/- 4%.

Is that error range tolerable?

Conversely when is it not tolerable?

Thank you.

Allan

 

[Jim E]

Just worked on a Chevy straight six and the range on the intake bolts was something like 18 to 24 pounds. Most every setting was in a broad range like that. I always stab for the middle number. Most of this stuff is pretty forgiving, has a range that works. On this intake it is plus or minus 3 pounds, what is that 15%.

You could always look around for a cal lab and have your wrench tested.

 

[Tom H]

Allan, I would bet that your "entry level" care made for greater accuracy than the "factory experience" and speed. I would be very surprised if the factory torque accuracy was better than +/- 20%. 20 years ago I helped a Chrysler lab set up a prototype cal system for calibrating the torque settings on their factory air powered wrenches. They said they had "no idea" how accurate the actual wrenches were. They were hoping to achieve something like 5% accuracy in actual use with their cal system. And keep in mind that even the torque level is just an attempt to achieve a certain tension level in the actual bolt. And the relationship of torque and tension depends heavily on the condition and lubrication of the threads. So just take your normal care, and use a decent wrench and you'll be fine.

Tom

 

[Wombat]

Allan

Your question on accuracy of torque wrenches sparked my curiosity. My Day Job involves measurement of rainfall, water levels and river flow and analysing that data. Knowing the reliability of the data is important.

I checked on the websites of a couple of makers of torque wrenches. Warren and Brown (warrenandbrown.com.au) claim +/-2% for their wrenches. Norbar (norbar.com) claim +/-3%. Norbar also claim that their wrenches meet various international standards for accuracy. The +/-4% you mentioned seems reasonable depending on the price.

One thing that I have learned from my work is to carefully read the claims of accuracy and see precicely what the percentage relates to. They either refer to a percentage of the range of the instrument or to a percentage of the actual measurement.

Another factor related to accuracy is repeatability. This is probably important for touque wrenches so that even if the bolts are 4% under torque, they are all the same.

Of course as Tom H says, there are so many other variables to generate the clamping load that a few percent error in the torque is probably insignificant.

Sorry for rambling, but I am developing an obsession for knowing the reliability of the numbers I am looking at.

 

[husky drvr]

Just for informational purposes;

my employer checks and calibrates all torque tools ( hand and air ) every three months. Each tool is set for its own unique torque. The tools have to be within +/-10% of set torque and repeatable.

We are ISO certified.

 

[SIVAllan]

A buddy will loan me one that rachets, makes a noise at the right spot, and is accurate to +/- 2%. I think that will be better than I can do manually...

Allan