machinists don't use torque wrenches

[spokewrench]

Except rear axle nuts, every fastener on my Aventon bike has a hex socket, but the supplied folding hex key set is terrible. More than 50 years ago, I discovered that the ISO hex keys that came with BMW motorcycles took the guesswork out of torquing, so I ordered a complete set. The H4, H5, H6, and H8 each supplied the specified torque to a fastener.

A couple of days ago I got a digital torque wrench with a 3/8" ratchet drive. I had assumed that an ISO key was designed to let you feel when you had reached a certain amount of torque. By using hex keys to apply torque to sockets on my new wrench, I discovered that I didn't have a feel for how much torque I was applying.

To double check the faster I'd torqued with allen keys, I used tape to make flags on hex bits. I'd insert the bit in the screw so that the flag was close to a reference point, use the wrench to back it off, and then to tighten to spec. In each case, the flag showed that the wrench had tightened the screw to the same angle as the hex key. I conclude that a proper hex key helps you torque correctly by feeling the "knee" in the increasing stiffness.

My neighbor is a machinist making truck transmissions at Eaton, a company that pulls in $25 billion a year. I proudly presented my new wrench. As soon as I handed it to him, he asked why I would want a torque wrench that small. That confused me. Didn't he ever torque anything to less than 60 Nm (45 foot pounds)?

He said they use only allen keys. During his training period, a supervisor would often check his work, telling him how close he'd come to perfection and explaining how the handle should feel. (I guess at the plant, torque wrenches are used only for fasteners so big that an allen key would have to be several feet long.)

With a torque wrench, an error in specs could mean a serious mistake. If machinists use allen keys, the tool maker need only specify bolts with the right size sockets. Torque requirements depend on friction, which can vary from what was anticipated. With an allen key, you're sensing the elasticity of the fastener, which is the point of torquing. I've never seen an allen key get out of calibration.

Privately, I love my torque wrenches, but if anyone might see, I'll use my ISO hex keys. I've got my reputation to consider!

 

[Stefan Mikes]

I was observing Specialized technicians and salesmen at work. Whatever they do on the bike (like setting the saddle height for a customer), they always use fixed torque wrenches similar to this one:

A 5 Nm fixed torque wrench.

These guys work with expensive bikes and e-bikes that do not belong to them. Working with torque wrenches is the part of avoiding responsibility if anything goes wrong. Bicycles are a way more delicate than trucks, don't you think so?

 

[spokewrench]

Three years ago I bought a torque driver that works on that principle. I tested it today with a strain-gauge driver. The first time, it clicked at 4.07. The tenth time, it clicked at 3.80. For the 8 between, it clicked between 3.85 and 3.90. The whole spread was only 7%: pretty good. For the middle 8, it was 1.2%: great!

Precise, but not at all accurate. When I hadn't had it long, it clicked at 4.8, so now it's 20% low, due to 3 years of compression on the spring. What's more, it was erratic when I tested it a few weeks ago. Friction affects these tools. Maybe there was a bit of internal oxidation the last time.

I think the issue is not how delicate a machine is but how much leeway there is between "tight enough" and "too tight." With metal bikes, I think most fasteners have lots of leeway. Some bike mechanics say they use torque wrenches only to avoid being blamed in case of a mishap, as you say. My seat post clamp has plenty of leeway. With an ISO H4 key, I can feel what I'm doing much better than with a bit or folding keys. When it's tight enough to keep the post from moving, I can feel that I haven't nearly taken up the elasticity in the bolt. It's doing its job, and that's tight enough for me.

If a bolt in a factory has to handle a load near it's strength, there won't be much leeway between tight enough and too tight, and there will be more at stake than the price of a bicycle. If machinists are like commercial pilots, tool makers are like airline pilots. Besides designing parts for bicycles and motor vehicles, tool makers take charge of factory machinists. I was surprised that the tool makers at Eaton still find allen keys better than torque wrenches.

 

[smorgasbord]

I previously worked at a off-road vehicle manufacturer. On the assembly line, they not only used torque wrenches, they used digital torque wrenches. They not only used digital torque wrenches, they used connected digital torque wrenches, and the torque applied to each bolt is stored in a database tied to the vehicle's VIN.
Now, not all bolts were torqued this way (eg dashboard trim pieces), but anything that mattered for safety was.

 

[spokewrench]

I believe the machinists like my neighbor build and maintain the machinery used for production. I hadn't thought of assembly workers. I'll have to ask him if torque wrenches are used there.

Henry Ford's assembly line sped up production but made work so boring that there was a high turnover. OTOH, it made tasks so simple that with attractive wages, he could grab somebody off the street to keep the line going.

If I had to to keep an assembly line going, I'd want a wrench that told an untrained worker how tight to turn each fastener. What's more, a connected wrench could let a supervisor pull up a list of how much torque an employee used each time and if he missed any fasteners.