Preventing the Weakest Link: To Grind or Not to Grind Your Pins

To Grind or Not to Grind Your Pins? When making field connections on your chain DO NOT grind your pins because this will only cause you more headaches in the upcoming months. When discussing chain, the old saying “Your chain is only as strong as your weakest link.” has never been so true and by grinding a pin you have just created a weak link.

As machining and hole punching processes get more precise we can use these processes to our advantage and add more value into the chain for our customers. Websters current processes allow us to reproduce a consistent and repeatable interference fit between the pins and the sidebar holes. With this repeatability, we solely rely on our press fit to keep the pins stationary in the sidebar. There are always a few exceptions to the rule based upon the geometry of the parts, but for the most part you will notice our pins with a full round end.

At this point you might be wondering what will happen if you do grind your pins. It can’t be that bad right? Wrong. By grinding the pins or reaming out the sidebar holes you are compromising the press fit that we have work so hard to achieve. This will then transfer the load bearing surface from the designed area, the pin outside diameter and the barrel/bushing inside diameter, to the unwanted area, the pin outside diameter and the thickness of the outside sidebars. Let’s take a look at a cut away view of a chain (Fig. 1) to determine what will happen if a pin loses its press fit in the sidebar. When a chain is designed, typically the rated working load of the chain is determined by the pin bearing area of the chain multiplied by the allowable bearing pressure of the materials being used. If we focus strictly on the pin bearing area, it is calculated by the pin’s diameter multiplied by the length of the bearing surface. In Figure 1 we have a 1” pin diameter with a 4-3/8” long bearing surface, as long as the pins are fixed/locked into the sidebars. If the press fit is compromised, then based upon the friction of the components, the pin will want to stay with the inner portion of the chain, the bushing/barrel, when the chain articulates around the sprocket. This will transfer the wear to the pin outside diameter and the two thicknesses of the sidebar. In the case of this particular example the pin bearing area as the chain was designed is 4-3/8 square inches , 1” x 4-3/8”. However by grinding the pins, the pin bearing area is now reduced to 1 square inch, 1” x (1/2” + 1/2”), which will drastically increase the rate at which this link will elongate. In Figure 2 a picture of a pin which has lost its press fit in the sidebar is shown. It is very evident where the wear points of the chain are, which is in an unwanted area.

To summarize, some of the major reasons the press fit of the pin into the sidebar is so valuable are as follows:

  1. The press fit between the sidebar and the pin is the main method we use to keep our components from spinning in their respective sidebar holes. This will keep the general chain wear on the surfaces designed to take the wear.
  2. The press fit also adds compressive stresses around the sidebar holes which improve the fatigue fracture resistance of the sidebar material around these holes.
  3. By having a good press fit the shaped/flatted pins and sidebar holes are not needed to help keep the pins rotating with respect to the sidebars they are pressed into.

By eliminating the shaped holes in the sidebars we have eliminated any sharp corners in the holes. This eliminates any additional high stress concentration points in the sidebar. The high stress concentration points can lead to fatigue fractures and chain failure over a period of time.

GrindingPins1 GrindingPins2

Brian Cline, Senior Design and Application Engineer

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