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Lost Insert on Roller Bit

Carbide Insert Lost on Piling Roller bit – A problem research

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Lost Insert on a piling roller bit is one typical problem, but not that frequently appear. 

We have mentioned this on the previous post “7 most Common Piling Roller Bit Failure & Causes”. At that post, we mentioned the lost insert mainly caused due to roller bit quality issue. 

Today, we have receive the client request to help analysis this issue which they encounter recently, to check the cause of this issue, whether if it is operational or that brand’s quality issue. 

Roller Bit Insert Structure

Before we go ahead to analysis the problem. Let us take a minute to understand more about the roller bit Insert Structure.

Roller Bit Cone Structure

Basically, the insert and cone body connected by the interference fit. It means it connected by the nature pressure between 2 material, Steel & Carbide. The Carbide insert is 0.05-0.08 mm later than the cone hole, and the hardness of 2 material much difference. So it can be fit by this method. There is close to third of five ( 3/5) of insert should root into the cone to guarantee its strength.

Interference Fit has some pro & cons. 

Pro:

There is no heat involved, not affect the heat treatment of the cone body. Cone body can keep higher mechanical strength.

Con:

Afraid of  extreme heat duration working.  If there is extreme heat ( the heat turns the steel into red), the thermal expansivity of carbide and steel is different. This different will make the number ” 0.05-0.08 mm” interference fit not qualified. 

All the possibilities
Check & Analysis

The good thing is that all this 4 causes can relatively easy checked. 

1. Interference Gap

Take a new roller bit, use the braze gun to heat the insert an hammer it out, then can check and measure the gap, to see if the number is 0.05-0.08 mm.

2. Hardness of Cone Steel 

For this case, this cause is not very possible. This can tell from the photos. If the steel is too hard, there should be cracks around the cone hole. If the steel is to soft, when the insert lost, the shape of the hole should be deformation. Not like the perfect hole on the picture. 

3. Submerge insert Height

Check whether the lost insert is  close to 3/5 merged into the cone steel. For our roller bit, the short insert outside height is 8.15 mm, but the total length is 19 mm plus.

4. Extreme Heat

Ask the operational manager, whether the roller bit turns in to red after it pull out from the hole. Check the roller bit bearing whether good or not. The rubber bearing can only stand for 120 degree -180 degree. If the roller bit stand big heat, all the rubber and bearing of the roller bit should be damaged. When turn the roller bit, the roller bit should stuck or lose. 

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