Aluminum Bolts used in a "slip-critical" case (cyclic loading & fa 3

[awcsag]

I have a situtation where a 1" steel plate is bolted to a 5/8" aluminum plate with 1/2" diameter 6061-T6 aluminum bolts with a 27 foot-pound torque (let's ignore the galvanic action for now). The plates are not allow to slip. 262 Locktite Threadlocker has been applied to the threads. The bolts are subjected to an 800 pound to 1200 pound cyclic shear load at about 240 cycles per hour.

At this point in time, the bolts are not loosening up. The connection is to last about 30 years.

Being a structural engineer, my belief is that over time the aluminum bolts will loosen up, because, the aluminum bolts are not suppose to be used as "slip-critical" bolts. Only ASTM A325-SC and A490-SC high strength bolts
are suppose to be used as"slip critical" bolts and "slip critical" bolts are suppose to be used in cyclic and fatigue conditions. To obtain an adquate margin of slippage of the connected parts, the minimum yield strength must be 54 ksi or greater. Aluminum bolts can not obtain this.
>
> Any comments?
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> Steve

 

[CoryPad]

The statement

"over time the aluminum bolts will loosen up, because, the aluminum bolts are not suppose to be used as "slip-critical" bolts."

is specious. Mechanical fasteners are governed by physics. In this case, you need to determine the input loads (forces, moments, temperature changes, etc.) and perform calculations for screw fatigue stress, external forces exceeding joint compressive force, etc. How have you determined that 54 ksi yield strength is necessary? If it is, and your screws cannot meet this requirement, then you may have a long-term problem.

 

[awcsag]

Answer to CoryPad's question to me: Reference, Guide to Design Criteria for Bolted and Riveted Joints, 2d Ed. by G. Kulak, J. Fisher & J. Struik, pg. 1, "In 1934 they (Batho & Bateman) reported to the Steel Structures Committe of Scientific and Industrial Research of Great Britian that bolts could be tightened enough to prevent slip in structural joints. It was concluded that bolts with a minimum yield strength of 54 ksi could be tightened sufficiently to give an adequate margin of safetly against slippage of the connected parts."

 

[CoryPad]

I see. Screw/bolt yield strength by itself is no indicator of joint performance. Rather, it is one of many factors that must be considered when designing fastened joints. For your specific case, here are some initial calculations:

Assumptions:

6061-T6 alloy according to ASTM F 468, 31 ksi (214 MPa)tensile yield strength and 30 ksi (207 MPa) ultimate shear strength

0.2 friction coefficient at the head and thread interfaces and joint faying surface

13 threads per inch (1.95 mm)

0.75 inch (19 mm) head outer contact diameter

0.55 inch (14 mm) hole size

Results:

27 ft-lbs (36.6 Nm) input torque will develop 3140 lbf (14 kN) preload

22 ksi (153 MPa) initial screw prestress

630 lbf (2.8 kN) will slip the joint

3960 lbf (17.6 kN) will shear the screw

It appears that you have a design problem.

 

[awcsag]

A solution that I have been promoting is to use 1/2" Dia. A325-SC (slip critical) bolts turned down (or necked down on a lathe at the shear plane) to 1/4" dia. The minimum diameter of A-325 high strength bolts is 1/2". Torque the bolts to 35 foot pound. Each bolt should should generate 4,165 pounds of pretension and shear at 5000 pounds.

Minimum slipage=0.2(4165)4 bolts = 3,332 pounds which is greater than 1,200 pounds. Coeff. of friction could be increased by roughening the surfaces of the mating parts.

Any comments?