Steering axis inclination - load on king pin post bushing

[seuf]

I would be grateful for any assistance

I'm attempting to calculate the load on the bushing in the kingpin post on this steering axle. I've uploaded a pdf of the assembly. The load would be perpendicular to the kingpin center line at the top and bottom of the bushing. I'm not certain how the figure this. Am I correct with what I have so far:

The machine is 1400 lbs at the rear. Two wheels so, 700 lbs on each wheel.

The load through the kingpin would be:

C = 700 lbf / sin15 = 2704.6 lbf

 

[Baluncore]

Welcome to PF.
Assuming that the kingpin thrust bearing caries load but provides no alignment control, the kingpin bushing will need to handle two forces due to torque from the offset wheel.
The kingpin will exert an outward force against the lower part of the bushing.
The kingpin will exert an inward force against the upper part of the bushing.
Those forces will be inversely proportional to the length of the bushing, reaching a maximum pressure at the ends of the bushing.

Steering is less sensitive to ground conditions when the kingpin centre line axis passes through the midpoint of the tyre contact patch on the ground. Why do you not do that?

 

[seuf]

Thank you for the response. I like the idea of the king pin center line intersecting the midpoint of the tire contact patch. Unfortunately, this is an existing design. That is something I will definitely implement into the Gen 2 design. Thank you. The issue is the existing bushing wearing prematurely. So, I'm trying to determine the forces on the bushing to select the proper type of bushing to replace the existing one. I was incorrect in figuring the force thru the kingpin center line, wasn't I? It should be:

C = 700/cos(15) = 724.6lbf - correct

 

[seuf]

I understand the forces acting upon the top and bottom of the bushing. but I am not certain how to figure those out. I understand the basic concept of sum of forces and moment arms. But am uncertain how to implement them in the instance.

 

[Baluncore]

The kingpin in the bushing is subjected to a torque about it's centre point of;
Torque = ( 700 lbf ) * ( "horizontal distance" between “centre of tyre contact patch” and the “midpoint of kingpin” ).

You can reduce failure of the bushing by:

Reducing the pressure per unit area on the bushing:
1. Lengthen the kingpin.
2. Increase the diameter of the kingpin.
3. Move the “tyre contact patch” horizontally closer towards the point below the "kingpin midpoint".

Changing the bush material / conditions:
1. Change bushing materials, to a higher pressure material.
2. Change the lubrication. Fit a grease nipple.
3. Use two part bushing with a lubricant reservoir between.
4. Use better seals to keep dirt and water from the bearing surface.
5. Change the bearing type from a bushing to two needle roller bearings.
6. Reduce the bushing clearance on the kingpin to reduce impact and dirt entry.

Study the mode of bushing failure.
1. Abrasion wear due to movement. Axial, or torque? Distribution?
2. Crush due to pressure. Constant, or impact?
3. Extrusion due to bushing material flow properties?
4. Loss of lubricant. Failure to maintain as specified?

 

[Baluncore]

Height and axis inclination are not really important here. Increasing the diameter of the wheel to shift the axis to intersect the present contact patch would not reduce the torque as it does not bring the patch horizontally closer to the midpoint. It is simply a scale change, requiring a bigger kingpin - bush assembly.

The relative horizontal distance between the kingpin midpoint and the tyre contact patch is now 82 units. The axis of the kingpin passes 25 units inside the contact patch. By moving the contact patch to intersect the kingpin axis, the torque on the kingpin will be reduced to (82-25)/85 = 67% of the current torque.


If we assume that the tyre contact patch originally intersected the kingpin axis on the ground surface, then we can say that, when the contact patch was moved out by some previous modification, the torque on the kingpin was increased by 50%. That would explain the bushing problems now appearing. That would suggest an investigation of the engineering change history of the assembly should be done immediately.

If the torque is now 50% greater than the original design, then to handle that increased torque would require a bushing area increase of 50%. You could increase the kingpin diameter by 50%, or increase it's length by 50%, or change both by say about Sqrt(1.5)=1.23, or 23% each.

Changing the bush material alone would require a 50% improvement in material compression properties. But you should also be concerned that the strength of the kingpin and supporting structures may now also be overstressed by about 50% above the original design values.

So, first check the engineering design changes to find out what changed. You will also need to investigate, did the wheel rim widen? or maybe the rim depth offset changed when the rim supplier changed?

 

[seuf]

Thank you again for your insight. This is the original design no engineering changes have been made. We have had no failures with the kingpin itself only the the bushing. The issue is to replace the bushing in 150 existing machines. My assumption is that the original material (Bronze SAE 660) could not handle the loads. But obviously I am uncertain of this until I know what those load are. Please see attached. I have calculated the loads at the top and bottom of the bushing using a moment about the center of the bushing. The "compressive stress" on the bushing would be the force Fa or Fb divided by one quarter of the surface area of the inside of the bushing correct. Knowing the compressive stress will help to determine a better material? View attachment AXLE BUSHING LOAD3.pdf

 

[Baluncore]

Your numbers appear to be of the correct magnitude.

For the kingpin angle of 15°. The thrust bearing will have a vertical component of 700. lbf * Cos(15°) = 675. lbf. The thrust bearing will have a horizontal component of 700. lbf * Sin(15°) = 180. lbf. Depending on the design of the thrust bearing, that 180. lb side force may need to be carried by the bushing.

My quick crude analysis is as follows:
7.62” = 0.194 m
Mass 700 lbs = 317.5 kg
Force = 317.5 * 9.8 = 3112. kN
Torque = 3112. kN * 0.194 m = 602 Nm
Kingpin diameter is 1.75” = 44.45 mm
Area of bush end is about 1” x 1.75” = 0.04445 sq m
Pressure is 602 N / 0.04445 sq m = 12 MPa
That is much less than the brass yield strength of about 200. MPa.
So the brass is not being crushed. The problem is abrasive wear.

Wear of the brass may be due to poor finish of the kingpin, dirt ingress, or lack of lubrication.

Does the kingpin have a poor finish, or might it be rusting and so causing wear of the bushing?
Maybe you will need to replace the kingpin, can you plate and polish the pin surface?
What lubrication improvement can you consider? A better synthetic grease?
You might be able to press or shrink a polished, stainless steel, thin sleeve onto the kingpin.