Fastener choice and why...

WildBill said:

I replaced the ones on my old Pathfinder with button head torx and lock+flat washers because I hated the screws, and the leaks. I didn't tighten them very much but got the biggest washer I could in hope it would add clamping area. The screws always seemed loose. Stopped the leaks for the next couple years.

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This, all of this...

Update, I got the screws removed/cover off, and as WildBill stated, at least one was kinda loose, none of them felt "torqued down" (factory max spec is 2.2 ft-lbs (this could be the reason for phillip--reduce temptation to over-torque?), probably could've removed them by hand if I'd eaten my wheaties this morn!

Also, as much as I'd like to ditch the phillips head in favor of something else, got a feeling me local hardware store is probably not gonna have a unicorn that is the correct length and has that "speed taper" (what is the proper term for that?) on the end:



May hafta give the OEM hardware a thorough cleaning and reuse. On a positive note, the Tekton #3 phillips 1/4 socket and GearWrench 10" 1/4 extension coupled to the Hart 1/4 90T ratchet (all recent purchases, "thanks" to GJ) worked like charms, no sinking feeling of rounding the head!

Thanks as always for all the replies and help!

This, all of this...Update, I got the screws removed/cover off, and as WildBill stated, at least one was kinda loose, none of them felt "torqued down" (factory max spec is 2.2 ft-lbs (this could be the reason for phillip--reduce temptation to over-torque?), probably could've removed them by hand if I'd eaten my wheaties this morn!Also, as much as I'd like to ditch the phillips head in favor of something else, got a feeling me local hardware store is probably not gonna have a unicorn that is the correct length and has that "speed taper" (what is the proper term for that?) on the end:May hafta give the OEM hardware a thorough cleaning and reuse. On a positive note, the Tekton #3 phillips 1/4 socket and GearWrench 10" 1/4 extension coupled to the Hart 1/4 90T ratchet (all recent purchases, "thanks" to GJ) worked like charms, no sinking feeling of rounding the head!Thanks as always for all the replies and help!

Why are Flange Bolts better than Standard Hex Products

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Washers have traditionally been used to protect the joint surface from damage during the tightening process and to distribute the load under the bolt head and nut. The stress under the face of a standard hexagon headed nut can result in indentation into standard strength steel surfaces. In recent years washers have started to be replaced by flanged headed fasteners for several reasons that will be discussed in this article. Prior to going into the details, lets look at some background.

Using torque to indirectly control a bolt&#;s tension is by far the most popular controlled tightening method. This is due to its simplicity and its widespread understanding by the majority of engineers.

The nominal torque necessary to tighten the bolt to a given tension can be determined either from tables, or, by calculation using a relationship between torque and the resulting bolt tension, or by testing. Friction that acts in the threads of the bolt and under the bolt head dissipates the majority of the tightening torque. For a free spinning nut, only about 15% of the torque is actually used to extend the bolt. The majority of the torque is used to overcome friction under the nut face and in the threads.

The way in which this can affect replacement fasteners is that the original equipment manufacturers (OEMs) specify the tightening torque based upon tests or calculations on the fasteners that are installed on the equipment.

Many OEMs specify flanged headed fasteners for a number of reasons. Using a replacement fastener not of the same type, strength and finish as that originally specified will lead to potentially serious problems since the torque-tension characteristics will be affected. That is, the preload generated by the tightening torque may be insufficient to prevent the joint or the bolt failing.

Flanged headed nuts and bolts have been developed to eliminate the deficiencies that have been found with the use of washers.

Standard washers are typically softer than the nuts and bolts that they are used with and can often plastically deform under the high compressive stress that they must sustain. For higher strength fasteners, this can lead to the washer dishing and indenting resulting in the clamp force being reduced significantly. An example of such a washer is shown in figure 3. Flange headed nuts and bolts do not suffer from this effect since the washer face is as strong as the bolt/nut itself.

It is not unusual for short grip length joints, with several interfaces, to experience a bolt tension reduction of up to 50%. A joint that may perform satisfactorily when flanged nuts and bolts are used may suffer problems when washers are inserted into the joint. Such a change can invalidate all the calculations and testing pertaining to the joint completed by the manufacturer.

Using a plain hexagon headed nut and bolt without washers will increase the surface pressure under the bolt head and nut face well beyond that which a standard structural steel surface can adequately support. The consequence of this is that the amount of embedding loss is significantly increased resulting in a higher than anticipated bolt tension loss. Essentially the risk is that for short grip length joints, the bolts can come loose without nut rotation occurring. The adverse effect that this can have on the joint&#;s structural integrity is obvious.

As has been explained, there are several reasons as to why flanged nuts and bolts are preferred over standard hexagon fasteners used with washers. The sensible approach is to replace like fasteners with like fasteners; unless you are feeling lucky.

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