Contact Us
My Disclaimer ie I donâ€t know what Iâ€m talking about so do not take any notice of what Iâ€ve written.
This describes my current knowledge (or lack of it) on this topic. I am not qualified in any way to offer this advice or recommendation. This information is sourced mainly or entirely from the internet. I have not validated this information or the sites from which it came nor has this article been reviewed by anyone with expertise in this area.
If you choose to use this information you do so at your own risk and should validate it with your own research from qualified and/or experienced experts in the relevant field.
If you identify something that I have written which is wrong in substance please let me know by this post.
6 April 2009 -My math in the Math - Its Easier... secton has been confirmed.
What is Torque?
Torque as lb/ft is the turning force applied by a weight attached to the end of a wrench one foot long (from the centre of the square drive).
The Actual Torque is calculated by applying a known weight on the wrench handle at a specific distance from the centre of the square drive. The torque that is applied is calculated by Weight X Distance (inches)/12 (inches).
Why Is Calibration Important.
First, if the manufacturer specifies a particular torque it is probably important.
Second, with the use of soft aluminium it is extremely easy to strip a thread with an uncalibrated wrench (like I did) potentially making a small job into a very big and possibly costly job.
Who Should Calibrate?
Everyone, especially those of us who donâ€t have professional quality tools.
The cheaper wrenches, typically bought in discount auto shops, have limited accuracy. At higher torques this is not such a problem ie 5lb/ft error at 50ft/lb torque is only 10% error but 50% error if you wanted 10ft/lb). In my case the indicated torque of 7ft/lb was actually 15ft/lb (ie more than twice required) causing the thread in my aluminium block to strip – ouch!!.
What You Need
Torque wrench, Vice, Metal bar for balance, Accurate Weight (or accurate spring scale), Tape Measure, Marker pen, String.
How You Calibrate With Weights.
Calibration is a matter of comparing the actual torque with that indicated on your wrench.
Example; calibrating with a 20lb weight.
Place your balanced wrench in the vice as shown in the diagram (make sure only the square drive has contact with anything to eliminate friction which will affect the calibration).
Set your torque dial to 20lb/ft.
Hang the 20lb off the wrench less than 1 foot from the centre of the square drive so that the wrench does not "click". Slide the weight away from the sqaure drive until the wrench first "clicks". You want to locate the point closest to the square drive along the wrench handle when the wrench will first “clickâ€.
Measure the distance from the centre of the square drive to the point on the wrench handle where the weight caused the wrench to first click.
Test with several different weights – especially lighter weights (10lb) as this is were errors can easily damage threads.
Note 1. The handle also applies force (mine applied 2lb force at 12 inches so the 20lb weight was actually 22lb) so you need to compensate or balance the wrench as shown in the diagrams.
Note 2. set the wrench indicator to the weight you use ie if its a 10lb weight set the indicator to 10ft/lb.
The Maths – Its Easier than You Think.
Note - this is updated math from initial post (validated 6 Ap 2009)
If your wrench (set at 20ft/lb) clicked when the 20lb weight was 14 inches from the square drive then your wrench is understating the torque it applies. The actual torque is 20lb X 14in/12in = 23.3 ft /lb.
So to use your wrench to apply 20lb torque you have to set it at 20 x 12/14 or 17.14lb/ft.
If your wrench (set at 20ft/lb) clicked when the 20lb weight was 10 inches from the square drive then your wrench is overstating the torque it applies. The actual torque is 20lb X 10in/12in = 16.7lb/ft.
So to use your wrench to apply 20lb torque you have to set it at 20 x 12/10 or 24lb/ft.
Even Easier Calibration – Horizontal Force Using a Spring Scale
Essentially this method is identical to Calibrating with weights. But it is done on the horizontal plane using an accurate spring scale to apply the force. As the plane is horizontal you do not have to compensate for the weight of the wrench and it is easier to get several calibrations weights (See diagram).
Beam Type Wrenches.
I have described the set up using a “click†type wrench. The same principle applies with beam type wrenches which have a pointer aiming at a scale on the handle. The pointer moves along the scale when the wrench handle bends when the torque is applied. With these you simply observe the indicated torque and compare it with the weight. It is easiest to hang the weight 1 foot from the centre of the square drive (eliminating any need for conversion) to give the true torque compared to the indicated torque.
This describes my current knowledge (or lack of it) on this topic. I am not qualified in any way to offer this advice or recommendation. This information is sourced mainly or entirely from the internet. I have not validated this information or the sites from which it came nor has this article been reviewed by anyone with expertise in this area.
If you choose to use this information you do so at your own risk and should validate it with your own research from qualified and/or experienced experts in the relevant field.
If you identify something that I have written which is wrong in substance please let me know by this post.
6 April 2009 -My math in the Math - Its Easier... secton has been confirmed.
What is Torque?
Torque as lb/ft is the turning force applied by a weight attached to the end of a wrench one foot long (from the centre of the square drive).
The Actual Torque is calculated by applying a known weight on the wrench handle at a specific distance from the centre of the square drive. The torque that is applied is calculated by Weight X Distance (inches)/12 (inches).
Why Is Calibration Important.
First, if the manufacturer specifies a particular torque it is probably important.
Second, with the use of soft aluminium it is extremely easy to strip a thread with an uncalibrated wrench (like I did) potentially making a small job into a very big and possibly costly job.
Who Should Calibrate?
Everyone, especially those of us who donâ€t have professional quality tools.
The cheaper wrenches, typically bought in discount auto shops, have limited accuracy. At higher torques this is not such a problem ie 5lb/ft error at 50ft/lb torque is only 10% error but 50% error if you wanted 10ft/lb). In my case the indicated torque of 7ft/lb was actually 15ft/lb (ie more than twice required) causing the thread in my aluminium block to strip – ouch!!.
What You Need
Torque wrench, Vice, Metal bar for balance, Accurate Weight (or accurate spring scale), Tape Measure, Marker pen, String.
How You Calibrate With Weights.
Calibration is a matter of comparing the actual torque with that indicated on your wrench.
Example; calibrating with a 20lb weight.
Place your balanced wrench in the vice as shown in the diagram (make sure only the square drive has contact with anything to eliminate friction which will affect the calibration).
Set your torque dial to 20lb/ft.
Hang the 20lb off the wrench less than 1 foot from the centre of the square drive so that the wrench does not "click". Slide the weight away from the sqaure drive until the wrench first "clicks". You want to locate the point closest to the square drive along the wrench handle when the wrench will first “clickâ€.
Measure the distance from the centre of the square drive to the point on the wrench handle where the weight caused the wrench to first click.
Test with several different weights – especially lighter weights (10lb) as this is were errors can easily damage threads.
Note 1. The handle also applies force (mine applied 2lb force at 12 inches so the 20lb weight was actually 22lb) so you need to compensate or balance the wrench as shown in the diagrams.
Note 2. set the wrench indicator to the weight you use ie if its a 10lb weight set the indicator to 10ft/lb.
The Maths – Its Easier than You Think.
Note - this is updated math from initial post (validated 6 Ap 2009)
If your wrench (set at 20ft/lb) clicked when the 20lb weight was 14 inches from the square drive then your wrench is understating the torque it applies. The actual torque is 20lb X 14in/12in = 23.3 ft /lb.
So to use your wrench to apply 20lb torque you have to set it at 20 x 12/14 or 17.14lb/ft.
If your wrench (set at 20ft/lb) clicked when the 20lb weight was 10 inches from the square drive then your wrench is overstating the torque it applies. The actual torque is 20lb X 10in/12in = 16.7lb/ft.
So to use your wrench to apply 20lb torque you have to set it at 20 x 12/10 or 24lb/ft.
Even Easier Calibration – Horizontal Force Using a Spring Scale
Essentially this method is identical to Calibrating with weights. But it is done on the horizontal plane using an accurate spring scale to apply the force. As the plane is horizontal you do not have to compensate for the weight of the wrench and it is easier to get several calibrations weights (See diagram).
Beam Type Wrenches.
I have described the set up using a “click†type wrench. The same principle applies with beam type wrenches which have a pointer aiming at a scale on the handle. The pointer moves along the scale when the wrench handle bends when the torque is applied. With these you simply observe the indicated torque and compare it with the weight. It is easiest to hang the weight 1 foot from the centre of the square drive (eliminating any need for conversion) to give the true torque compared to the indicated torque.
