This is the last of our three-part post on chromed shafts.
In order to extend service life, by resisting surface wear on the rod, the rod surface needs to be hard. It also needs to be relatively smooth, so as not to tear up the seals.
And the rod needs to be straight. In most cases, chrome is an ideal surface coat for Hydraulic Rods. Hard Chrome may be better. IHCP is better still.
The finish however, cannot be so bright that it will not attract and hold the lubricant. Inadequate lubrication is just one of many potential contributors to shaft failure, chromed or not. You might to consider the following information, for a broader scope on this topic:
Fatigue Failure and The Importance of Design;
I have always felt that approximately 80% of shaft failures in heavy industrial maintenance applications are related somehow to fatigue. The following article found in Machinery Lubrication magazine compliments that supposition.
In his article on cylinder rods, Brendan Casey states that it is recorded that approximately 25% of hydraulic rod failures are design related. In hydraulic cylinder applications, one in four shafts fails to provide adequate service life. Bent rods make up a considerable portion of failures. It is important to clarify the straightness tolerance your vendor is proposing to supply.
Rods that are of insufficient diameter and insufficient strength will often bend in service. Once the rod bends, excessive load is placed on the seal. That results in premature seal failure. If a rod is bent, and use of a larger diameter rod is not practical, then the tensile of the rod must be increased. Induction hardened material (IHCP) offers a significant boost in strength.
A NOTE ON BAR ENDS
When full bars are chromed, most mills hold the bars by the bar ends and vertically coat them. In those instances, the bars are not fully chromed the length of the mill random bar. Several inches on each end will be bare. This is generally not trimmed off when random bars are sold. It is recommended that you specify “trimmed ends” when you order random length chromed bars. Probably a good idea to specify this on even cut-to-length material, even though most vendors provide that service as a default.
This completes our post on chromed rods. If you want to pursue information gathering on this type of material, you may want to do some additional research on; Black Nitrided Rods. That process is showing improved service life in similar applications.
-Howard Thomas, June 11th 2021
