Short answer:
Don’t worry about linear extensions for screwdriver-style torque wrenches. The only way they’ll mess with the torque value is if they have a built in clutch with a lower torque setting.
Longer answer:
By definition, a screwdriver-style torque wrench extension twisting will not change the torque value. Torque is simply how much of a twisting load the system is carrying, and it’s resisted by the resistance of the screw threads (which depends on among other things the clamping of the joint). You don’t have torque losses unless things are trying to move relative to each other (bearings, driveshafts, gears, etc., or fast motion relative to a fluid). Static things like a length of rod or a socket/bolt head interface don’t try to move relative to each other.
NOTE: you DO have variable and often-inconsistent torque losses through things like 90 degree screwdriver heads, torque shafts within flexible tubes, and universal joint sockets, because you need to overcome friction to make all of these work. If you need to use one, do yourself a favor and put a calibrated slip clutch (like the one Borka Tools makes) on the driving end and use your ratchet of choice on the driven end.
The typical “extensions mean you have to change your torque” comes from socket-wrench-style torque wrenches with crowfoot extensions. When you use one of those wrenches with an extension at the head of the wrench, some of the side load you impart on the wrench can also be also imparted on the extension, and therefore your applied torque is increased. Also, you lose the ability to ratchet this style of torque wrench with an extension.
Three ways to get the right torque on the bolt, if you need a crowfoot for access or you can’t use a socket:
- Use a screwdriver-style torque wrench with a crowfoot extension, and twist it only - don’t push. It’s a pain in the ass but sometimes for small fasteners it’s the only option you have.
- With the extension inline, set the torque wrench to Torque=TorqueDesired*D/(D+d) where D is the distance from the middle of the grip to the middle of the drive head on the torque wrench, and d is the distance from the middle of the drive socket to the drive head or socket or jaws in the direction of grip-to-head on the torque wrench. Yes, d can be zero or negative!
- My preferred solution, taking advantage of d being zero: with the extension at 90 degrees to the wrench so it looks like an L if you were to look down on the joint, set the torque wrench to the desired torque value and make sure you only push in the same direction as the extension points.
On torque flex:
This is negligible for extensions, but does matter for bolts that are long relative to their shank diameter. When we apply torque, we’re really trying to put a known amount of linear clamp load into the bolted interface. If a bolt is long, and the threads and head are far from each other, you still apply the correct torque to the threads, but the torque being carried in the bolt makes it twist and get shorter, which increases the load on the bolted interface, which increases friction in the threads, which means that your torque isn’t as useful. When you stop applying the torque, the bolt untwists, gets longer, and relieves the load on the interface, so it can be loose even if the torque is “right” for that bolt size. If you merely increase the torque setting to get the right preload after the bolt relaxes, you can easily pop the head off of the bolt while you tighten it, or damage the material under the bolt head.
