| Author | Message | ||
     Alexander Lynch New User Username: lex_lynch Post Number: 11 Registered: 07-2020 |
Bentley MK V1 1950 B111 JN I am replacing the front damper bushes and would like to know what the running clearance is between them and the nose cone as the two parts of the damper have been bolted together and are ready to be line bored. | ||
| ChristopherCarnley Unregistered guest Posted From: 5.62.43.249 |
Contact Andrew Baldwin in Stamford, he is a professional rebuilder of these parts. abclassics@gmail.com (Message approved by david_gore) | ||
| Alexander Lynch New User Username: lex_lynch Post Number: 12 Registered: 07-2020 |
Thank you Chris I will contact Andrew, Regards Alex | ||
| Bill Vatter Experienced User Username: bill_vatter Post Number: 136 Registered: 09-2004 |
Not clear what you mean by "bushes." Do you mean the Babbitt in the gear and front hub that fits on the nose of the damper? These run on different places on the front of the crankshaft and have different diameters. They are fed oil through the center of the crankshaft and oscillate relative to the crankshaft a small amount as the damper is absorbing torsional crankshaft vibrations. I don't think the diametral clearance there makes much difference. It needs to be tight enough to stop lots of oil passing through and causing loss of oil pressure. That defines max clearance. Loose enough to turn freely without binding defines the minimum. If that was my damper, I would test fit those pieces separately on their respective journals and feel the fit. If you can just barely feel a slight wiggle, that's probably a thousandth or two, and I would say that's about right. I perceive you're looking for something more scientific, but I haven't seen anything with an actual dimension specified. I also haven't heard of it ever having been a problem. The damper can have lots of trouble in various forms, but I don't think loose bushings are one of them. | ||
| Alexander Lynch New User Username: lex_lynch Post Number: 13 Registered: 07-2020 |
Thank you Bill, the reply I received from Andrew was - "The general rule for bearing clearances - The old rule of thumb is to provide .0007? to .001? of bearing clearance for every inch of shaft diameter in a stock engine. Consequently if the crankshaft has 2 inch diameter journals, the rod and main bearings should be assembled with .0015? to .002? of clearance" with thanks to Andrew Baldwin. | ||
| ChristopherCarnley Unregistered guest Posted From: 5.62.43.248 |
It is not as though the bushes are in a constant rotational state, and the bushes are always a tight fit both the outer wheel and pinion are always difficult to remove. Depending upon the finish of the bushes and the crankshaft nose, a fitting clearance of a maximum of .001" is appropriate. (Message approved by david_gore) | ||
| Bill Vatter Experienced User Username: bill_vatter Post Number: 137 Registered: 09-2004 |
Not applicable to this specific situation, but for journal bearings operating in a challenging application like big-end and main bearings, the amount of clearance can be critical, and different for different bearing materials. (Different types of Babbitt, aluminum, bronze, Halls metal, etc.) While the diameter thumb rule might be helpful as a general guide, I would not use it for setting clearances unless it was all I had. Even then, not meaning any disrespect to your friend, I would research the theory behind that thumb rule to be sure it is a good thumb rule and to see if there are any contrary views on that subject. Your application is not highly loaded and variation isn't going to be critical. | ||
| Bill Vatter Experienced User Username: bill_vatter Post Number: 138 Registered: 09-2004 |
The damper bushes turn only a very small amount, less than two degrees of rotation. Hydrodynamic lubrication is not established like is critical in those other places. For hydrodynamic lubrication, you need adequate clearance for the oil to do its thing. Here, that's not needed. Therefore, tight is all right. If it turns, that's enough. 0.001 or less, as Chris indicated, will work very well. | ||
| Alexander Lynch New User Username: lex_lynch Post Number: 14 Registered: 07-2020 |
Thank you all, I have indicated to the engineer to allow 0.001 clearance to the bearing. Today I spent cleaning out the oil ways in the the crankshaft after grinding and tomorrow I will fit the end caps. | ||
| ChristopherCarnley Unregistered guest Posted From: 5.62.43.184 |
Alex, Please do not overtighten the 2BA bolts and then have the end projecting into the crankpin. Always fit new thin aluminium washers. (Message approved by david_gore) |
