Shackle bolt torque

[Author nameAuthor names are only available to PAS members.]

Has anyone seen the actual torque specifications/procedure P-A used for re-installing the spring shackle bolts after greasing? This was recommended to be done by the P-A dealer but I haven’t had much luck finding one.

This may have been one of the reasons for failure of the Fafnir ball bearings. My lower rears on the front springs are totally trashed-some of the balls have actually turned to rust dust while the uppers and 2 out of 3 fronts are fine. This car was parked in 1948 with 50000 miles and it is clear the pins and balls had failed seriously from wear when parked – probably before the corrosion happened. So much for the original design being adequate – unless they were badly over-torqued by someone when re-greasing. The Fafnirs are angular contact balls to take side loads, which like tapered rollers are sensitive to preload. PASB 97-5 does mention fitting them up tight and backing off 1/4-1/2 turn as done with wheel bearings which is probably close to correct. A quicky calculation indicates if the 1/2″ bolt were torqued up to the 50 ft lb level the compression of the steel tube spacer would be on the order of .003″ and easily preload the bearings well over 1000 lbs – that would exceed their axial load capacity. Sorry for the dissertation, but it would be great if someone has run into the “official” procedure. The whole system relies on the precise length of the shackle spacer and the torqueing of the bolt. There might be a bunch of over-torqued shackle bolts out there.

[Author nameAuthor names are only available to PAS members.]

The instruction in PASB 97-5, applies only to the forward front spring bearings. The toothed nut is to be adjusted with a pin spanner wrench.

The rear shackle adjustment is with the proper pin length (and spacer washers as required). The bolt does not preload the bearings, and should tight.

Given the condition of your bearings, I would suggest replacement with modern needle bearings; these are available from Dave Murray.

[Author nameAuthor names are only available to PAS members.]

Dave indicates he no longer supplies the shackle rebuild kits. At any rate, the spring shackles must take significant side load when the car turns. A needle bearing is great for vertical loads but does not react side loads, therefore I assume the old rebuild kit had provision for simple thrust washers. I have considered refitting with Timken rollers (which could fit), but coming up with a suitable preload method might be complicated. I will be working on a rebuild design to take side loading into account. I’m looking at chrome or nitrided pins with some very high load capacity permanently lubricated bushings.

There is some wear and brinneling on the ends of the Fafnir pins on the good bearings, indicating possibly the Fafnirs were designed to bottom out before overloading the balls, however fitting up the rear shackle assembly with its lower pin but balls removed showed the pins do not bottom out before the balls on the upper bearing take a significant preload. On my original units as I found them, tightening the assembly finger tight is enough to take out all end play, a mere 5 ft lbs – 1/8 turn – starts to measurably create drag on rotation, 25 ft lbs stiffens rotation up considerably indicating significant preload on the balls. Bottom line, I have no doubt the original assembly can easily preload the bearings. 5-10 ft-lbs torque is probably the limit – at least on this one example, the cotter pin a vital part of the assembly. The article reproduced in PASB 92-6 indicates no shims, – but also ‘impossible” to over tighten. I don’t believe that part.

The lubricant recommendations say “because of the necessity of adjusting the shackle bearings after lubricating….” Would like to know what that procedure was.

Thanks for responding, Jim

[Author nameAuthor names are only available to PAS members.]

The needle bearing rebuild kits use the end of the new shackle-pin bottoming in the needle bearing cup for side load or end load. The needle bearing cups are from U-Joints. In the normal application for these needle bearings. the cup bottoms on the cross pin of the U-Joint.

Greg L.

[Author nameAuthor names are only available to PAS members.]

Correction, and sorry for spewing some mis-information. I just re-checked the fit-up of the same spring shackle bearings while measuring for my new shackle pins and now I can torque the bolt to 40 ft-lbs and the shackle rotates freely with barely detectible endplay. i.e. no apparent axial pre-load. It isn’t totally repeatable, but may have had the felt washers installed the first time throwing me off. My apologies to Fafnir.

Jim

[Author]

Posts