What Are the Limits of Torque Amplification in Mechanical Torque Multipliers?

[Machinekraft]

What is the maximum torque amplification that can be achieved using a mechanical torque amplifier?

 

[Bystander]

Today I tried to remove the rear axle nut from my car. It was really torqued on there, I couldn't break it with a 1/2 drive breaker bar 2 feet long. So I supplemented that with an eight foot cheater bar. All I accomplished was to sheer off the 1/2 lug where the breaker enters the socket.

A look at an auto forum taught me that people have broken even 3/4 breaker bars trying to loosen this nut. You need an impact wrench or a dedicated tool. It's supposed to be torqued to 250 ft lbs, but some shops go overboard and people have found their axle nut torqued up to 400 ft lbs.

What's the torque required to break things?

 

[Machinekraft]

What's the torque required to break things?

The axle nut,tends to tighten further while the wheel is running. The torque on the nut increases by the taper provided on the face of the nut. So to loosen the nut, a impact wrench is used to overcome the friction of nut and wheel. In case of bolt few blows on the bolt head relieves the jam. It is impact that loosens the hold.

 

[Bystander]

Which takes care of Zoobyshoe's question, but we've yet to address yours. Archimedes made a statement in that regard, as well. Have you something specific in mind?

 

[Dennis C]

Mechanical torque amplifiers vary in size and ratings.
We use industrial amplifiers for very large fasteners because you could never produce the torque needed for the clamping force required by hand. By very large I mean diameters greater than 2".
I would say it broke because the nut was seized to the stud. That is the reason it is a "covered" or an acorn nut because of the environment.
Not sure what diameter stud is on your hub, but a failure of the tooling or fastener with an 8 foot cheater instead of the thread loosening
isn't because of the tightening torque. The thread would strip or fastener break at that value.
400 lb ft of torque is the value used when you get up to 7/8" diameter stud / bolt with a course thread.
You are not running 7/8" diameter stud. When installing your lug nuts lubricate the threads and reduce tightening torque to
obtain proper clamping force. There are many charts that show proper tightening torque vs. diameter, pitch and thread quality.
Getting the torque even is very critical for wheels also, you can actually produce an out of flat (warped) disk (brake) from unequal torque (per 1 wheel)

"The torque on the nut increases by the taper provided on the face of the nut."
I don't understand this.
And the lug nuts won't tighten as you turn the wheel ( what about the right side of the car, wouldn't they loosen?)

 

[zoobyshoe]

Mechanical torque amplifiers vary in size and ratings.
We use industrial amplifiers for very large fasteners because you could never produce the torque needed for the clamping force required by hand. By very large I mean diameters greater than 2".
I would say it broke because the nut was seized to the stud. That is the reason it is a "covered" or an acorn nut because of the environment.
Not sure what diameter stud is on your hub, but a failure of the tooling or fastener with an 8 foot cheater instead of the thread loosening
isn't because of the tightening torque. The thread would strip or fastener break at that value.
400 lb ft of torque is the value used when you get up to 7/8" diameter stud / bolt with a course thread.
You are not running 7/8" diameter stud. When installing your lug nuts lubricate the threads and reduce tightening torque to
obtain proper clamping force. There are many charts that show proper tightening torque vs. diameter, pitch and thread quality.
Getting the torque even is very critical for wheels also, you can actually produce an out of flat (warped) disk (brake) from unequal torque (per 1 wheel)

"The torque on the nut increases by the taper provided on the face of the nut."
I don't understand this.
And the lug nuts won't tighten as you turn the wheel ( what about the right side of the car, wouldn't they loosen?)

It wasn't a lug nut, it was an axle nut. 36mm diameter. The car is a 1972 Beetle. You have to take this nut off to get the rear brake drum off. A shop could get it off with an impact driver with no difficulty. The home mechanic tend to break tools trying.

 

[Dennis C]

Well, that explains a lot.
Sorry for the confusion on my end.

 

[Baluncore]

What is the maximum torque amplification that can be achieved using a mechanical torque amplifier?

The limitation is the strength of the square socket drive. For nuts above 1” you should be using a ¾” or 1” square drive socket. An impact wrench requires a socket with higher quality steel, capable of withstanding the impact. There are two types of torque multiplier.

The simple breaker bar or lever will give you enough torque to break the square drive if you extend it with a tube.

The rotary torque multiplier contains a stack of planetary gears. The torque ratio is usually in the range from 25 to 100 times. Greater torque multiplication will break the square drive or requires too much time to take up the slack in the gear train.

When a big nut must be tightened or released a hollow hydraulic jack resting on a bridge over the nut is used to stretch the bolt. The nut can then be removed easily without high friction between the nut and the washer surface. The problem then is that it needs a longer exposed bolt thread.

 

[zoobyshoe]

The limitation is the strength of the square socket drive. For nuts above 1” you should be using a ¾” or 1” square drive socket. An impact wrench requires a socket with higher quality steel, capable of withstanding the impact. There are two types of torque multiplier.

The simple breaker bar or lever will give you enough torque to break the square drive if you extend it with a tube.

The rotary torque multiplier contains a stack of planetary gears. The torque ratio is usually in the range from 25 to 100 times. Greater torque multiplication will break the square drive or requires too much time to take up the slack in the gear train.

When a big nut must be tightened or released a hollow hydraulic jack resting on a bridge over the nut is used to stretch the bolt. The nut can then be removed easily without high friction between the nut and the washer surface. The problem then is that it needs a longer exposed bolt thread.

This is the cheaper of the two dedicated tools made to remove the Beetle nut:

http://www.airheadparts.com/vintage-vw-parts/miscellaneous-tools/36mm-rear-axle-nut-removal-tool

You put the hex hole over the nut then bang on the edge sticking out with a sledge. People say it works great. It's not so useful for re-torquing the nut when you're done, though.

This tool is better, you can get the nut off with a 3/8 drive ratchet, and torque it back on, but it's more expensive:

http://www2.cip1.com/ProductDetails.asp?ProductCode=C13-16-9602

This youtube shows the latter tool in action: