The slewing ring roller race on our 1931 R&R 40-ton breakdown crane ADRR95207 (MP3) needs adjusting and our crane team is looking for advice or ideas before we carry out the work this coming Sunday.
Adjustment is carried out by turning a large round nut that's located on the centre post immediately beneath the Travel drive's bevel gear, in a most difficult position to access.
The nut has two holes on its circumference, located 90 degrees apart. As it stands, one hole faces forward and the other faces to a side of course. The holes are each tapped 7/8 BSW and one of them is fitted with a hexagon-headed setscrew to lock the nut in place. The nut and the side-facing hole can just be seen in the following photo, taken prior to the start of the recent crane restoration. The setscrew in the other hole is not in view here.
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Centre Post Nut - Alex Chamberlain photo.jpg [ 76.82 KiB | Viewed 8807 times ]
The setcrew, pictured below, is steel with a copper-alloy tip. Our first question is: "Why does it have a non-ferrous tip?". The markings on the tip show that it engaged with the shaft via its circumference, not its end.
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Setscrew.jpg [ 71.26 KiB | Viewed 8807 times ]
As can be seen, the setscrew is bent, which may indicate that it was used to turn the nut sometime. That would be back in BR days as no adjustment has been carried out in preservation.
It has become clear that we need the crane in steam when carrying out the adjustment, and first the jib needs raising, with probably some weight on the hook to counterbalance the tailweight so as to bring the rollers into contact with the upper and lower paths all round.
To tighten the nut, we envisage screwing a threaded bar or bolt into one of the holes and slewing the crab clockwise until part of the crab framework comes into contact with the bar. Further slewing will compel the bar to turn with the crab, so tightening the nut.
Our second question: "Is the above procedure valid for a Ransomes & Rapier breakdown crane?". Roger Cooke has established that Coles used a similar method for at least one of their road mobile cranes but it would be useful to know if this is a valid procedure for our much larger R&R crane.
Third question: "Would there be several available counterbores in the centre post able to accept the end of the setscrew?". (By "counterbores" I mean recesses of any kind, including keyway-like, bearing in mind that the nut will move down the post as it is turned.)
Finally: "What sort of roller-to-path clearance should we aim for?".