excavater boom joints all loose line boring ?

[fastline]

High quality pins are typically made from medium carbon steels, or chrome-moly steels. The core of the pin is typically 80-100-ksi yield strength. The outer 0.100" is induction-hardened to between 45 and 50 HRC. For extra wear resistance, and protection from the elements, they are sometimes plated in chrome (72 HRC). The pin has toughness for impact-resistance, while the case-hardening provides applicable wear resistance. If the pin were through-hardened (like bushings are) the pin would fail from impact-loading (brittle fracture) in short order.

High quality bushings are typically made from air-hardening tool steels (similar alloy to what iron worker dies are made from). The bushings are machined from annealed round-bar, then heat-treated (through-hardened with light-temper) to between 50-55 HRC. Note that bushings can still fail from brittle-fracture, but it takes appreciable abuse. The hassle with these types of bushings is that the bore interference-fit must be nuts-on (normally requiring line-boring operations). Large bushings are often installed with shrink fit (liquid nitrogen bath). Smaller bushings can be pressed.

If you do the math for a typical excavator/dozer pin joint, the compressive stress easily exceed 80,000 psi, which is beyond yield for most steels. Non-case hardened 4140 pins (30 HRC typical) will not last nearly as long as case hardened pins (extreme-pressure grease is your friend).

A good source for induction-hardened chrome-plated round bar for DIY pins (both US and metric sizes) is CRConline.com. The normal application for this rod is hydraulic cylinders, but this stuff works great for pin joints also - provided you don't have to drill any grease holes through the induction-hardening. If you need to drill a grease hole, you need to grind the hole through the induction-hardening, to expose the softer core of the steel - which can then be drilled/machined. This type of rod is generally considered non-weldable (too high of carbon content - welding will compromise the heat-treatment at best, or initiate cracks at worst).

Just curious where your data is coming from? It does seem to align with what I would expect in those designs but I like to ask. I'm sure OEMs have toyed with the toughness/hardness to figure out what's needed. Obviously a pin fracture is simply not acceptable. I would also personally want to make the pin the primary wear item as it needed to maintain some ductility anyway.

I cannot say I've ever run into a pin with plastic deformation defects which would point to the softer side of the curves.

 

[Entropy1]

Plastic deformation happens when people try to use ordinary cold-rolled steel for pins. Chrome-moly steels are typically heat-treated & tempered to 30 HRC, which makes the steel tough & strong (4140 is a popular machine gun barrel alloy). However 30 HRC is still pretty soft for extreme pressure metal-to-metal pin joints. If the pin diameter is large enough (more specifically - a large enough surface-area between the pin/bushings), you can get away with non-case-hardened pins - for a long time anyway. Case-hardened pins with through-hardened bushings will always take the most abuse & last the longest.

A typical bushing that's through-hardened & tempered to 55 HRC will have around 280-ksi tensile strength. The steel that the bushing is pressed into will have around 70-ksi tensile strength (playdough by comparison). The bushing distributes the applied load-bearing-stress that would normally gall & yield the base material. However, if a machine is abused, the excess applied-stress can crack bushings, and/or deform bushing holes out-of-round.

Or if grease is not used, even hardened components are susceptible to galling. When a bushing seizes onto a pin, the OD of the bushing becomes the new slip-interface, and the machine's bushing hole will rapidly deform & go out-of-round.

 

[Entropy1]

The factory bucket pins on my Komatsu PC-128 are not case-hardened. They were worn down by the previous owner by almost 0.100" diametrically - very visible and very appreciable wear. I suspect they are the machine's original pins. But would you really want to replace just the pins?