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About Chromed Rods Part Three of Three

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

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About Chromed Rods – Part Two of Three

Continuing from our last post, we are discussing chromed rods. When they are applied correctly, chromed Rods typically extend service life compared to standard carbon and alloy bars. Variables relative to those materials would be the steel type, actual hardness, and the type of hardness (surface or throughout). The benefits of chromed rods address sliding surface abrasion and corrosion.

In actual service, however, even sliding abrasion applications are not just limited to sliding abrasion. Small bits of debris (tramp elements) find their way onto the exposed rod and then get pulled through the seal (intended to keep tramp elements out), between the rod and the cylinder. The motion of the rod is often interrupted by impact causing shock and twisting or bending of the rod.

Deciding between IHCP and Hard Chrome HP & Too great of a surface finish has a downside.

Since chrome plating is most often applied to a rod surface very thin (thousandths), the importance of a surface hardened sub-straight should not escape consideration. Where severe impact and gouging may be experienced, a hardened sub-straight will resist damage. You would think that, to be on the safe side, everyone should always order the IHCP material. The problem is, surface hardened sub-straights may be furnished at a hardness of 60 to 62RC (approximately 600bhn). Not everyone has machinery sufficient to handle that hardness. Drilling, cutting, machining may be a problem.

The thin chrome “skin” covering the rod is easier to penetrate when the sub-straight is soft. Various fabrication processes are readily accomplished. Smaller shops will find fabrication processes easier.

It is also important to always remember that high hardness in steel (carbon or alloy surface or throughout) also affects ductility. Application consideration must be given to the propensity for fracture.

CHROME SURFACE FAILURE
Abrasive related chrome surface failures often damage seals. When chrome gets very thin due to wear, it cracks and peels, exhibiting a razorblade sharp projection that often curls up, like the bark on a Birch tree. This type of failure is often experienced where there the surface of the shaft is subjected to severe temperature change (furnace). That projection tears up seals. In applications were liquid may escape, such as sludge from a barge pump, the contamination could be catastrophic. Read as “expensive”. In those applications, some alternative (such as utilizing hardened stainless shafts) should be considered.

Our last post of this three-part series will cover, lubrication, bar cut-ends, and dent bars.

 

-Howard Thomas, May 6th 2021

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