Torque is the tendency of a force to cause or change rotational motion of a body.
It is to set a standard so a screw or bolt is not under tightened which will result in failure from loosening, or over tightening which can result in breaking or stripping of the screw. The values determined are set in place to keep a constant force on an object to keep from moving.
2 main factors determine the torque specs used. 1 st is the size of screw and thread pitch Common size is an M5 x.8 which is M (Metric sizing 5 ( diameter of the screw) and.8 which is the space in between threads.
Next is the type of material used to make up the screw. A rule of thumb is the higher the grade of screw the higher the torque spec and therefore a greater force can be applied. Please note that different plating types used affect the torque values.
An example of these forces is a clamp screw is used to keep the pylon adaptor in place and resist rotational forces. An M5 x.8 bolt with a rating of 12 Nm does not allow the same forces applied as an M6 x 1 can achieve of 15 Nm
Another example are the 4 grub screws which we use to set our alignment and hold the Pyramid in place. The rule of thumb still applies for material grade, pitch and plating.
Common Torque Wrenches What are some of the common Torque Wrenches used and how do they effect the outcome of achieving the specifications set out by the Mfg.
These wrenches can range from 2 Nm to 25 Nm with a very low rate of accuracy!!! NOT available in preset Although they only need servicing a few weeks a year with careful maintenance they can last 30+ years. Benefit is a firm handshake and a strong arm.
This wrench is commonly found in hardware stores and depending on the make may have a +/- 4 – 5% accuracy rate. These usually have a larger incremental value in the lines and going over 1 or 2 lines make a huge difference. Not recommended for applications where accuracy is very important
These wrenches come in wide range of settings from 4 – 20 Nm and up to 500 Nm. Important that all wrenches have some sort of validation or testing. Higher degree of accuracy as they let the user know when they have hit the required setting by an audible clicking sound.
These wrenches are basically fool proof as they are already set at a designated setting. Coupled with an audible clicking sound make these the ideal types of wrenches. These types are also usually certified and tested to meet or exceed international standards for Torque.
Why do our bits keep breaking? We tried to find out some of the factors why these occur on a sometimes regular basis. Our findings are first based on the force required to loosen screws torqued to spec at 15 Nm on a Titanium Adaptor. We determined that breakage occurred more often on loosening that tightening.
We first examined 4 different types of grub screws all torqued to spec with 242 loctite at a neutral angle setting. We found that there was very little variance in the amount of torque required to loosen these screws.
We then tried the same screws at a higher degree of angle setting to see the effect it would have.
We found on average it took 7.908938032 to 8.473862177 Nm to break friction. Well this doesn’t seem to be enough to break a 4mm hex bit.
Next we decided to try a different type of adaptor. Maybe the material selection has an affect on the value?
So we tried an Aluminum Adaptor set at approx the same angle with the grub screws provided which were probably a mild steel. No Loctite was used. WOW!!!! We found on average that it took 11.298482902 to 12.993255338 Nm to break friction
Well we’ve gone this far let’s try SS using the same specs as we did for the Aluminum. We found that it took on average 10.733558757 to 14.123103628 Nm to break friction. Now I think were on onto something!!!
What does it take to break a 4mm 3/8” drive hex bit??? We tested 4 different makes of hex bits to see at what value did they fail. First we tried a Signet 4mm Bit and it failed by shearing off clean at 27.11635897 Nm Then we tried an Insert Bit commonly found in any hardware store.
The cheap Insert Bit shattered sending a small shard flying at 25.98651068 Nm Then we tried a German made product Hazet and it sheared off at 27.11635897 Lastly we tried a Proto and it finally failed by stripping and twisting at 33.89544871 Nm.
From this we can conclude that there are some other outside factors that contribute to these bits failing since the values for breaking friction are to low. Some of these are but have not yet been tested are: Amount and type of loctite used.
Foreign bodies getting into the threads like sand and dirt. Lastly and possible the most plausible is rust or seizing of the threads due to fluids.
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