athomas
10-27-2006, 10:33 AM
I was having a close inspection of my valve and its operation the other day, when I noticed my bumper had begun its normal shreading process. For those that don't know what it is, the urethane bumper of retro valve mags disintegrates when used with a level 10 bolt. Almost all valves equiped with the level 10 bolts do it, but why. I decided to investigate.
I think I have discovered the cause and effect of this phenomenon.
Design:
The bumper sits against the front of the valve chamber to protect against the impact of the returning bolt by providing an energy absorbing urethane layer for the bolt to hit. The bumper is held in place by a small groove at the base of the powertube. The bumper fits snug around the powertube and sits quite nicely in this groove to prevent the bumper from moving forward with the bolt.
Bolt and Bumper size:
The inner diameter of the bumper is slightly smaller than the inner diameter of the bolt. This allows the bolt to move back and forth on the powertube, and keeps the bumper anchored tight against the powertube to prevent movement with the bolt. The outer diameter of the bumper is similar in measurement but slightly larger than the outer diameter of the bumber. This ensures that the bolt maximizes the contact area of the bumper while trimming excess weight. The original level 7 bolt diameter at the back was full width of the back of the bolt.
Problem:
The problem begins when the back of the bolt impacts the bumper. The bumper compresses as it should. When it compresses, it attempts to spread out towards the path of least resistance. Since the outer diameter of the bumper is larger than the outer diameter of the bolt, the compression of the bumper material forces the bumber to squeeze out towards the edge of the bolt. It can't move towards the center because it is against the powertube, so it spreads towards the outer diameter. This outward spread pulls the entire urethane bumber towards the circumference. The inner edge of the bolt provides a pinch point on the inner edge of the bumper. The inner edge of the bumper against the powertube does not compress because there is no bolt material pressing on it. So, the inner edge of the bumper is held in place as the outer edge is pushed out. This stresses the urethane material and over time with continuing use, causes tearing and eventually leads to complete failure of the bumper.
The level 7 bolt didn't suffer from this because the back of the bolt was larger in diameter than the bumper. The bumper material couldn't migrate out because the back of the bolt was at the same depth and provided no relief for the stressed material.
solutions:
One solution mentioned here on AO was to glue the bumper onto the front of the valve chamber. This actually worked by preventing the bumper material from being able to move towards the circumference. By reducing the amount of movement, we reduced the stress and thus reduced the tearing of the urethane.
A total solution would be to have Airgun designs widen the back of future level 10 bolts so that the circumference of the contact area was larger than the circumference of the bumper to prevent the bumper material from trying to migrate out. This would act the same as level 7 bolts.
A theoretical solution for those that don't want to glue their bumpers, would be to trim the outer diameter of the bumper so that it is slightly smaller than the diameter of the contact area of the level 10 bolt. This would theoretically provide the same effect as the level 7 bolt by providing constant pressure evenly across the bumper material and not allowing the material to squeeze out. I would like to test this theory myself and call upon others to as well. If it works, then it may save us some frustration.
Comments?
I think I have discovered the cause and effect of this phenomenon.
Design:
The bumper sits against the front of the valve chamber to protect against the impact of the returning bolt by providing an energy absorbing urethane layer for the bolt to hit. The bumper is held in place by a small groove at the base of the powertube. The bumper fits snug around the powertube and sits quite nicely in this groove to prevent the bumper from moving forward with the bolt.
Bolt and Bumper size:
The inner diameter of the bumper is slightly smaller than the inner diameter of the bolt. This allows the bolt to move back and forth on the powertube, and keeps the bumper anchored tight against the powertube to prevent movement with the bolt. The outer diameter of the bumper is similar in measurement but slightly larger than the outer diameter of the bumber. This ensures that the bolt maximizes the contact area of the bumper while trimming excess weight. The original level 7 bolt diameter at the back was full width of the back of the bolt.
Problem:
The problem begins when the back of the bolt impacts the bumper. The bumper compresses as it should. When it compresses, it attempts to spread out towards the path of least resistance. Since the outer diameter of the bumper is larger than the outer diameter of the bolt, the compression of the bumper material forces the bumber to squeeze out towards the edge of the bolt. It can't move towards the center because it is against the powertube, so it spreads towards the outer diameter. This outward spread pulls the entire urethane bumber towards the circumference. The inner edge of the bolt provides a pinch point on the inner edge of the bumper. The inner edge of the bumper against the powertube does not compress because there is no bolt material pressing on it. So, the inner edge of the bumper is held in place as the outer edge is pushed out. This stresses the urethane material and over time with continuing use, causes tearing and eventually leads to complete failure of the bumper.
The level 7 bolt didn't suffer from this because the back of the bolt was larger in diameter than the bumper. The bumper material couldn't migrate out because the back of the bolt was at the same depth and provided no relief for the stressed material.
solutions:
One solution mentioned here on AO was to glue the bumper onto the front of the valve chamber. This actually worked by preventing the bumper material from being able to move towards the circumference. By reducing the amount of movement, we reduced the stress and thus reduced the tearing of the urethane.
A total solution would be to have Airgun designs widen the back of future level 10 bolts so that the circumference of the contact area was larger than the circumference of the bumper to prevent the bumper material from trying to migrate out. This would act the same as level 7 bolts.
A theoretical solution for those that don't want to glue their bumpers, would be to trim the outer diameter of the bumper so that it is slightly smaller than the diameter of the contact area of the level 10 bolt. This would theoretically provide the same effect as the level 7 bolt by providing constant pressure evenly across the bumper material and not allowing the material to squeeze out. I would like to test this theory myself and call upon others to as well. If it works, then it may save us some frustration.
Comments?