Understanding Bolt Torque vs Clamping Force
 

When the Model "A" was produced, there were no torque wrenches in use at the factory. The first car company to use torque wrenches was Chrysler in the mid 1930s. The torque wrench became available on the retail market in 1938. Being an engineer and knowing the pains taking design of structural joints, it is questionable when torque tables are arbitrarily applied to the Model "A."

We all know that tightening a bolt causes a clamping force, but how much is the clamping force? The attached table provides the amount of clamping force. So when you see a book that says torque to XXX Lbs-Ft, understand how much force is being applied, and does it pass your "giggle test."

Re: Understanding Bolt Torque vs Clamping Force
 

Torque value is used to place the fastener in tension after maximum clamping force is achieved.Placing the fastener in tension is the best way to insure the assembly doesn't degrade from heat or vibration.Torque value is determined by testing,causing fastener failure through assembly. The human body is very capable of 'learning' torque values and remembering them,mechanic 'feel' is amazingly accurate given enough repetition.

Re: Understanding Bolt Torque vs Clamping Force
 

It's my understanding that this "tension" is achieved by a specific amount of bolt/stud stretch and that the torque spec is the torque required to achieve a specific amount of stretch. This is also, to my understanding, why critical bolts/studs ( heads on high compression/high performance applications) should never be reused. They should only be "stretched" once.

Re: Understanding Bolt Torque vs Clamping Force
 

that chart is for unlubricated steel bolts ---it would be different for plated bolts, and different platings
in the old days there was snug, tight, and good and tight ---snug the oil pan bolts, tighten the head nuts, get the axle nut good and tight, the length of the wrenches determined the torque many times, I used the original tool kit wrench to tighten the head nuts last time it was off,

Re: Understanding Bolt Torque vs Clamping Force
 

Bob,

The attached chart shows the clamping force capability of the fasteners based on preventing "yield" of the fastener. The yield of the fastener is directly proportional to the "grade" or quality of the fastener and/or application.
It is a really good starting point, shows both dry and lubed torque values.

What the chart cannot factor in is the material of the assembled parts.
Usually in discussion is the head/gasket/block assembly.
The gasket creates a compressed material that is directly affected by the clamping forces applied. What most probably are not aware of is the distortion caused in the deck surface of the block when attempting to clamp too tightly, the result is inconsistent clamping and usually a leaking gasket.

This is a good topic of discussion, enjoy the 4th, John

Re: Understanding Bolt Torque vs Clamping Force
 

All this tech-talk makes my head hurt!

Re: Understanding Bolt Torque vs Clamping Force
 

My Cummins diesel requires the head bolts be torqued to 60 ft. lbs., then marked and turned an additional 120 degrees to get to the proper stretch. They do not require replacing the bolt each time it is stretched.


My Ford 7.3 diesel just gives a torque number and requires that the bolts be replaced each time they are torqued.



It is my assumption that the Cummins bolts are more resilient (like a spring) and the Ford bolts stretch to provide the correct tension, but do not return to original length. I have heard of connecting rod bolts that are not torqued, but measured before installation and tightened until they reach a specific length. They have a certain maximum length at which point they are replaced. The number of times they have been tightened is unimportant.


My point is, it's not like buying a baseball cap, one size does not fit all.

Re: Understanding Bolt Torque vs Clamping Force
 

Its called 'turn to torque,caterpillar used that spec as well..torque wrench to a value then turn 3 'flats' or 120°..we would run a 2 man team rolling in bearings in the field..set the torque then turn 3 flats with a 3/4 impact...not kosher but it got the job done..

Re: Understanding Bolt Torque vs Clamping Force
 

The torque that a particular bolt, etc is capable of is the design maximum and is not necessarily the best torque for the application per post #5, and the material. An aluminum part is going to be torqued different than say a steel part for the same application.

Re: Understanding Bolt Torque vs Clamping Force
 

What you are saying is the assembled items are put in compression by the fastener,giving the same result as putting the fastener in tension,
locking of the assembly by load is the desired result..

Re: Understanding Bolt Torque vs Clamping Force
 

Fasteners are classified in two categories. They can be either tension or shear and applied torque depends on which category it is. In aviation, there are three acceptable methods of torque up depending on the application. The most common is still the torque wrench.

The next is stretch torque where the fastener length is measured loose then tightened until it reaches a specific stretch length. These type fasteners are generally high tensile strength types that have their full strength potential at the stretch specification. These fasteners are also reusable if still in serviceable overall condition. Sometimes the nut threads get stretched causing too tight a fit so that would require replacement of the nut. Some of these types have an initial torque requirement to check for excessive yield of the bolt prior to final stretch. It's the only way to see if it meets specs for the design. The fastener is replaced if the mechanic stretches it too far.

The final type of torque up is relatively new and was developed for the military to simplify maintenance requirements. This is the wrench arc torque method. The engineers developed this method through testing of each fastener to find out how much the nut had to be turned after the fastener is drawn up snug but not yet torqued. Each different size and type of thread has a different amount of degrees it has to be turned to reach the desired torque. If it is 45 degrees for example, you snug the nut till there is no shake or movement by hand then you turn it another 45 degrees to reach the torque spec. It's simple and it works. The military currently uses it more than the civilian world but it's just a matter of time before it will end up to some degree in both worlds.

With the helicopters, the clamping force isn't always a all that much since most fasteners are small to reduce weight and the torque will be in the lower inch pound settings. It's where the shear loads increase where the fastener strength comes into play. Main rotor blade bolts have to take a lot of shear load so they are mostly stretch torqued for full fastener load capability. Some tail rotor blade & hub fasteners are also in this category but the rotor is a lot smaller so the centrifugal loading is a lot less.

The big load on the early Fords is the rear axle on that old tapered shaft with key application.

Re: Understanding Bolt Torque vs Clamping Force
 

Typically, joints are designed with bolts torqued to 75% of their yield strength to reduce the alternating stress range.