#1 Differential Pinion Shaft Collapsible Spacer
Posted: Sat Feb 23, 2019 10:16 pm
Hello All,
On other occasions in this Forum, I've voiced my opposition to the use of the Collapsible Spacer to set bearing Pre-load. Following are some of my thoughts on the matter.
The collapsible spacer is positioned between the set of two bearings. The logic applied to this arrangement is that the natural elasticity of the shaft is used to keep the system tight (the reason behind tightening nuts/bolts to a specified torque), but not apply that amount of torque as a pre-load force to the bearings. In effect, the collapsible spacer tends to keep the bearings apart and soaks up most of the torque so that its not all applied to the bearings. Using the Pinion Shaft and Bearing System as an example, if the 120lb/ft to 140lb/ft torque specified were to be applied to the bearings as a Pre-load, the shaft would require a lot of force to rotate and the system would fail in a short time.
So the hypothesis is that the collapsible spacer will collapse just the right amount so that the cones of the Taper Roller Bearings come into intimate contact with their respective cups, just prior to the specified torque being reached and that the remaining torque to be applied will put the bearings under the correct Pre-load. That all sounds fine if you say it quickly, but for that to happen, the following criteria must be satisfied, as per the original design:
1. The starting length of the spacer must be just right.
2. The wall thickness of the spacer must be correct.
3. The compressive strength of the material must be the same.
4. The amount of Pre-collapse must be correct.
Even the distance between the bearings will affect the result. The distance between the bearings has the potential to vary due to the cone setting distance of the pinion.
The above may have worked well enough back when the original equipment was available, but typical of much of the current After Market parts being produced, I’ve found these collapsible spacers to vary extraordinarily. In tests carried out using Pinions that wont be used again, I’ve observed results in excess of 200lb/ft torque before the bearing cones and cups made contact (Zero Endplay – Zero Pre-load).
Prior to the collapsible spacer being introduced, the Pre-load was achieved by shimming and the measurement of the torque required to rotate the Pinion; a very predictable method. With differentials that use the collapsible spacer, I replace it with a solid spacer and employ the tried and true method of determining the Pre-load by checking the effort required to rotate the pinion shaft.
When using the collapsible spacer system, I’d bet that very few would check what Pre-load the bearings are actually under, but blindly follow the instruction and torque the Companion Flange nut to the specified torque. One well known repairer and supplier of parts here in Australia when asked if they ever checked the Pre-load of the pinion bearing when the collapsible spacer is used, replied with pride that they check this on every diff worked on. Asked how they checked it, replied that they check the Pre-load with a dial indicator. At this point I declared them to be unique in the World, being able to measure less than no movement (my colloquial term for Pre-load) with a dial indicator. This is the type of chump that can be working on your car and actually charging for the privilege.
The additional time taken to adjust the Pre-load using a Solid Spacer is infinitesimal in the whole scheme of things and well worth it, knowing that the bearings actually have the correct Pre-load.
Regards,
Bill
On other occasions in this Forum, I've voiced my opposition to the use of the Collapsible Spacer to set bearing Pre-load. Following are some of my thoughts on the matter.
The collapsible spacer is positioned between the set of two bearings. The logic applied to this arrangement is that the natural elasticity of the shaft is used to keep the system tight (the reason behind tightening nuts/bolts to a specified torque), but not apply that amount of torque as a pre-load force to the bearings. In effect, the collapsible spacer tends to keep the bearings apart and soaks up most of the torque so that its not all applied to the bearings. Using the Pinion Shaft and Bearing System as an example, if the 120lb/ft to 140lb/ft torque specified were to be applied to the bearings as a Pre-load, the shaft would require a lot of force to rotate and the system would fail in a short time.
So the hypothesis is that the collapsible spacer will collapse just the right amount so that the cones of the Taper Roller Bearings come into intimate contact with their respective cups, just prior to the specified torque being reached and that the remaining torque to be applied will put the bearings under the correct Pre-load. That all sounds fine if you say it quickly, but for that to happen, the following criteria must be satisfied, as per the original design:
1. The starting length of the spacer must be just right.
2. The wall thickness of the spacer must be correct.
3. The compressive strength of the material must be the same.
4. The amount of Pre-collapse must be correct.
Even the distance between the bearings will affect the result. The distance between the bearings has the potential to vary due to the cone setting distance of the pinion.
The above may have worked well enough back when the original equipment was available, but typical of much of the current After Market parts being produced, I’ve found these collapsible spacers to vary extraordinarily. In tests carried out using Pinions that wont be used again, I’ve observed results in excess of 200lb/ft torque before the bearing cones and cups made contact (Zero Endplay – Zero Pre-load).
Prior to the collapsible spacer being introduced, the Pre-load was achieved by shimming and the measurement of the torque required to rotate the Pinion; a very predictable method. With differentials that use the collapsible spacer, I replace it with a solid spacer and employ the tried and true method of determining the Pre-load by checking the effort required to rotate the pinion shaft.
When using the collapsible spacer system, I’d bet that very few would check what Pre-load the bearings are actually under, but blindly follow the instruction and torque the Companion Flange nut to the specified torque. One well known repairer and supplier of parts here in Australia when asked if they ever checked the Pre-load of the pinion bearing when the collapsible spacer is used, replied with pride that they check this on every diff worked on. Asked how they checked it, replied that they check the Pre-load with a dial indicator. At this point I declared them to be unique in the World, being able to measure less than no movement (my colloquial term for Pre-load) with a dial indicator. This is the type of chump that can be working on your car and actually charging for the privilege.
The additional time taken to adjust the Pre-load using a Solid Spacer is infinitesimal in the whole scheme of things and well worth it, knowing that the bearings actually have the correct Pre-load.
Regards,
Bill