The samples were stroked varying amounts, depending on whether or when they failed, as well as time constraints. Samples 1, 2, and 3 were tested under ideal conditions (no side loading). None of these samples exhibited any seal, push rod, or bore wear, nor did they have any leaks after 20,000 strokes. No configuration was determined to be better than another. However, sample 2 lost about 40% of its black-oxide coating.

Samples 4 and 5 were subjected to side loading and did not have any design modifications to reduce side-loading damage. Consequently, both exhibited severe side-loading damage similar to that found on field returns. Figure 5 shows the bore damage to sample 4.

The significant bore wear is due to the push rod rubbing on the same area as a result of a repetitive side load. This damage causes aluminum grit from the bore wall to enter the system. Dimensional deformation causes the seal to not seal properly, resulting in brake fluid bypassing and leaking. Sample 5 exhibited similar damage.

Both samples showed that the action of the push rod wearing on the bore wall created the side-load damage. The steel push rod, significantly harder than the aluminum, made it the largest contributing factor to the wear observed.

Samples 6 and 7 had modified designs to eliminate the damage to the bore wall from the push rod. Sample 6 was tested using a push rod with a polytetrafluoroethylene (PTFE) cap on its end, applied by using PTFE heat-shrink tubing to the end of the push rod. The resulting cap was 0.015-in. thick. This sample was inspected after 10,000 strokes, at which point the PTFE cap had split into two pieces, and fine aluminum grit was present in the system.

The cylinder bore showed evidence of side-loading wear, although it appeared that the PTFE cap delayed the onset of bore wear. Sample 8 was tested with a machined nylon 6/6 cap on the push rod end. Because the nylon cap is significantly thicker than the PTFE cap, the push rod was turned down so that no dimensional interferences occurred with all the other components current design.

This sample was run for 30,311 strokes and then inspected. Minimal wear occurred to the nylon cap, there was no aluminum grit present in the system, and the cylinder bore showed little to no wear. The nylon cap proved very effective in eliminating side-loading damage and preventing aluminum particles from contaminating the system. 

Conclusion

Side loading is the most common damage observed on master-cylinder field returns and can lead to premature product failure. In an effort to improve the master-cylinder product, several samples of existing and new designs were tested.

Under ideal conditions — particularly linear loading — the existing master-cylinder design is sufficient. However, subsequent testing under side loading revealed that the current design is susceptible to damage. We found that the major cause of side-loading damage is from the steel push rod wearing on the aluminum cylinder bore.

The wear patterns created in testing are quite similar to those observed on field returns. The design and testing of a nylon cap on the rod end showed that side-loading damage essentially can be eliminated. Implementing this solution will result in a superior product with a longer life.

Bill O'Rourke is General Manager and Co-owner of Jet Products LLC, Lexington, KY.  For more information, call him at (630) 605-5944, email worourke@jetproductsllc.com, or visit www.jetproductsllc.com.

Jet Products LLC manufactures custom engineered small hydraulic cylinders, master cylinders, hydraulic hose reels and brake products — all made in the USA. In early 2013, the company completed a substantial expansion of its production capacity and moved its headquarters and product development to Lexington, KY earlier this year.