How accurate does a torque wrench have to be?

-
DartVadar

DartVadar

Well-Known Member
Joined
Apr 3, 2013
Messages
1,293
Reaction score
39
Location
Alberta
I'm about to start assembling my motor and was wondering about the accuracy of the average click type torque wrench.

Mine isn't that old, and hasn't been used too often. But I was curious about it's accuracy. I measured the length from the ratchet to the handle, and got a bucket of water with a known weight, and calculated that it should take 96 foot pounds to click ( middle to top end of its range) Setting up the bucket on the handle it clicked at about 90-91 foot pounds, so the accuracy seems around 5 percent. Does that number seem right? Now there are some variables that have an impact on the accuracy of my test, but it's still pretty close.

Would this wrench be accurate enough to use to assemble my motor?
 
DartVadar said:
I'm about to start assembling my motor and was wondering about the accuracy of the average click type torque wrench.

Mine isn't that old, and hasn't been used too often. But I was curious about it's accuracy. I measured the length from the ratchet to the handle, and got a bucket of water with a known weight, and calculated that it should take 96 foot pounds to click ( middle to top end of its range) Setting up the bucket on the handle it clicked at about 90-91 foot pounds, so the accuracy seems around 5 percent. Does that number seem right? Now there are some variables that have an impact on the accuracy of my test, but it's still pretty close.

Would this wrench be accurate enough to use to assemble my motor?
Click to expand...

click wrenches are the very basic torque wrenches and are not very accurate. You can give the same wrench to 10 people and they will all get different results when pulling them. The reason is they measure a minimum torque. They let loose if you will when a minimum torque is achieved and then trust the operator to stop pulling. They do not tell you if something is over torqued or what the final torque is on a fastener, just that a minimum torque has been applied. That being said is it accurate enough to build you engine, it depends on what type of build you are doing and what is the torque range of the fasteners you are using. In many cases yes, it is ok, in other cases where you need tight tolerances then no. Some High end race shops do not use click wrenches or beam wrenches for fastening, other manufactures use click wrenches to audit fastened bolts and make sure the residual torque on the fastener is at a minimum value.

(I use to teach courses on torque and fastening)
 
your method for checking the torque wrench is an acceptable home-way to measure precision.
You may want to repeat the test 3 to 5 times and get an average error and an estimation of the accuracy.

Most click wrenches are more precise and accurate towards the middle of the tools capacity and less precise and accurate towards the ends.

So vary the weight and check at the low - center - and high end, plot the average error and you will see how linear the tool is.

5% would be well within acceptable tolerances for normal engine assembly.
If you find the tool consistantly reads for example 4 lbs light, you could boost the target by 4 lb..

Click torque wrenches are handy and are an absolute must when your working in a position where you can't see the scale.. When working in environments where the scale is easily visable,,like building an engine on a bench,, for criticals like head-main-rod bolts,,I assemble with a click torque wrench and go back and verify with a beam style torque wrench. The beam wrench is accurate and precise to the resolution on the scale unless the pointer or tool is damaged.
 
When you are finished be sure to back off the adjuster to zero otherwise the spring will take a set and affect it's accuracy.
 
my68barracuda said:
your method for checking the torque wrench is an acceptable home-way to measure precision.
You may want to repeat the test 3 to 5 times and get an average error and an estimation of the accuracy.

.
Click to expand...

x2
 
Let me tell you my horror story, brand new craftsman click wrench, 30 years ago, building a 440 and I snapped one of the short head bolts on the bottom of the head rebuilding a 67 Magnum. Took it to work and our calibration lab, thing was 30 lbs short, ie. 70 on dial was 100! So they melted the lead from the end, calibrated the spring and put it back together. Now I do my first steps, say 20 and then double check with my beam wrench to check the cal. If the wrench doesn't come with a calibration certificate, it's not calibrated. Period.
 
I will do the test a few more times to make sure that I was accurate. I didn't see a calibrations sticker on my 1/2 drive wrench, but there was one on my 3/8 drive wrench, it was from 99 though lol, tested it the same way and it seemed even more accurate than the other one. But if some don't use click or beam wrenches for assembling parts what do they use?

And that's the margin of error with my measurements, I knew the weight of the water in the bucket because I measured it and used that to convert it into the total weight including the bucket, I also weighed it on a scale (not overly accurate I don't think), but because it was pretty full I spilt some, and there are slight variations when I measured the water as well as the location of the weight on the wrench handle.

So to add how accurate are beam wrenches? Because I like the idea of torquing with a click type and confirming that the torque is correct.
 
From what I am reading you did not calculate they boyency of water in it?
 
5% is acceptable for a clicker. Most that I send in for calibration come back within 2-3%. Beam style wrenches are only as accurate as the user. The scale marks are quite close together and are hard to read consistantly. The dial wrenches are just about the same. The new digital wrenches are quite accurate as long as they are calibrated correctly. (the same as all torque wrenches)
I truly believe that as long as a torque wrench is reasonably within spec (within 5%) the consistency of the torque is far more important than the ultimate accuracy. A head bolt doesn't fail if it is torqued 1 or 2 ft/lb tighter than spec but if it is a ways out of calibration you may be in trouble. All the new torque specs are torque to yield which purposely stretches the bolt to the proper torque. Get your torque wrench calibrated, back the adjustment down when storing it if it is a micrometer style, and use it properly. Don't forget to lightly lube the fasteners before tightening. (don't assemble dry and don't drench the bolts either) That will give a more accurate torque. tmm
 
toolmanmike said:
5% is acceptable for a clicker. Most that I send in for calibration come back within 2-3%. Beam style wrenches are only as accurate as the user. The scale marks are quite close together and are hard to read accurately. The dial wrenches are just about the same. The new digital wrenches are quite accurate as long as they are calibrated correctly. (the same as all torque wrenches)
I truly believe that as long as a torque wrench is reasonably within spec (within 5%) the consistency of the torque is far more important than the ultimate accuracy. A head bolt doesn't fail if it is torqued 1 or 2 ft/lb tighter than spec but if it is a ways out of calibration you may be in trouble. All the new torque specs are torque to yield which purposely stretches the bolt to the proper torque. Get your torque wrench calibrated, back the adjustment down when storing it if it is a micrometer style, and use it properly. Don't forget to lightly lube the fasteners before tightening. (don't assemble dry and don't drench the bolts either) That will give a more accurate torque. tmm
Click to expand...

I will double check the numbers that I got but it seems like they are still pretty accurate, so it's safe to use them I guess?

And I had another thread about bolts, new bolts from what I understand arent torque to yield, because of they were wouldn't you only be able to use a bolt once? The bolts are torqued while still in their elastic stage.

I have arp bolt lube for my rods and head bolts, and regular oil on my mains (stock bolts) so that's all taken care of.
 
If I knew it was over 1 ft pound off I would and have tossed into the nearest trash can, dont forget that using extensions throws the readings off and as mentioned above using lubes on bolts/nuts/washers does also.
 
bbrroowwnn said:
exactly how accurate is that Karl? enlighten us...LOL
Click to expand...

The kind used by Cal-tech physicists and NASA engineers....

It's an industry term....


The video sucks, but it's the only one with this scene....

[ame]http://youtu.be/jvJHQM9tijE[/ame]
 
Lol that post makes a bit more sense now.

I went out and did a few more tests on my big and little wrenches. I think my weight was a bit off last time, I varied the weight by a few pounds each test to see a bit of a range (I'll maybe do some more extremes too) I did three total tests, the average with the big wrench was 3.44% and the little one was 3.37%.

The big one was bought about 2 years ago and not used often, and the little one was calibrated in 99 but not used often and stored right as far as I know, is the need to calibrate based on use or time? Because from those numbers both seem pretty accurate, also have to take into account the margin of error with my testing.
 
Make sure that you are measuring to the centerline of the socket....
 
For aircraft use it's about plus or minus 3%. I think it's important that the torque wrench is consistent. You could always go get it checked by a professional because even some top-of-the-like snap on torque wrenches become unable to calibrate and essentially defective.
 
If you want to be accurate, use a beam wrench....
 
I used to calibrate them for the USAF. Most were +/- 3% but some were more like the real low inch pound screwdrivers. They had to be temperature stabilized for 24 hrs at 70F and no more than 20% HUMIDITY before calibration. If I remember right they were calibrated at 10, 50 and 75% of their scale and also the highest setting. Before USE THAY HAD TO BE BROKEN 3 TIMES AT THE TORQUE THEY WERE GOING TO BE USED AT! I always thought that was kind of bull, because you took it out of the shop to the flightline and it could be 0 F out there or 130f and 80% humidity and what happened to the calibration if it is out of the testing conditions? If it was dropped they had to be re calibrated. Hope it helped and not confused you.
 
krazykuda said:
If you want to be accurate, use a beam wrench....
Click to expand...

So a beam type torque wrench would be good to ensure that the torque settings are accurate when building my motor? It's on a stand so I can easily see the dial.what type do people usually do to assemble a motor?

I guess I am probably just being overly cautious because I really don't want to have to pull this motor apart again...
 
Thanks for the input everybody, and from what I understand 3 percent is pretty good for what I am doing, and as far as I know the wrench is pretty accurate, nothing has made me think that it is out of whack anyways.

I found a few places around me that can calibrate them as well, but wondering if I have to.
 
You must log in or register to reply here.
-

Back
Top