Clyde_Wanderer
Well-Known Member
I had an oil leak at the rear face of my Hurth HBW 50 gearbox for quite a while and decided to remove it for repair.
The leak was comming from where the idle gear pivot shaft showed through the alloy casing.
On stripping the box I discovered that this pivot shaft is an interference fit in the box and had a circlip on its outer end.
The shaft was loose in the casing and had quite a bit of movement which was letting oil out around it.
Obviousley the radial loading on the bevel idle gear was the cause of it loosening and there was no way of tightening the shaft with the present design, as the shaft was a parrallel shaft which meant that if it was pulled towards the back of the box then the thrust clearence on the gear faces would have been reduced, so I came up with a modification to effect a permenant solution.
Above pic shows the original shaft fitted in the box, (idle gear removed) and can be slid in and out for its entire length easily by hand.
The above pic shows the original shaft (right) with its needle roller brg, and my new modified design which has a larger needle roller fitted into the gear and a stepped portion on the shaft, which is drilled and threaded at the end so when the shaft is pressed into the box a washer and bolt can be screwed into its outside end allowing it to be drawn tight into the box without effecting the gear thrust clearance, as the step on the shaft tightens against a thrust plate (not showen) which is fitted between the inside face of alloy box and idle gear.
I do think this is a better design than the original, and am surprised that Hurth never done this.
It cost me £55 all in to have the gear machined out, the new shaft made and the purchase of the bearing, a lot cheaper than buying a new or s/h box which would eventually have the same problem, bearing in mind there is only about 2-300 hrs use on the box.
Above shows new shaft in box with the stepped face pressing against the thrust plate and below shows outside end of shaft where washer and bolt are used to pull shaft outward.
The leak was comming from where the idle gear pivot shaft showed through the alloy casing.
On stripping the box I discovered that this pivot shaft is an interference fit in the box and had a circlip on its outer end.
The shaft was loose in the casing and had quite a bit of movement which was letting oil out around it.
Obviousley the radial loading on the bevel idle gear was the cause of it loosening and there was no way of tightening the shaft with the present design, as the shaft was a parrallel shaft which meant that if it was pulled towards the back of the box then the thrust clearence on the gear faces would have been reduced, so I came up with a modification to effect a permenant solution.
Above pic shows the original shaft fitted in the box, (idle gear removed) and can be slid in and out for its entire length easily by hand.
The above pic shows the original shaft (right) with its needle roller brg, and my new modified design which has a larger needle roller fitted into the gear and a stepped portion on the shaft, which is drilled and threaded at the end so when the shaft is pressed into the box a washer and bolt can be screwed into its outside end allowing it to be drawn tight into the box without effecting the gear thrust clearance, as the step on the shaft tightens against a thrust plate (not showen) which is fitted between the inside face of alloy box and idle gear.
I do think this is a better design than the original, and am surprised that Hurth never done this.
It cost me £55 all in to have the gear machined out, the new shaft made and the purchase of the bearing, a lot cheaper than buying a new or s/h box which would eventually have the same problem, bearing in mind there is only about 2-300 hrs use on the box.
Above shows new shaft in box with the stepped face pressing against the thrust plate and below shows outside end of shaft where washer and bolt are used to pull shaft outward.