Coefficient of Friction between Lifting Trunnion and Wire Rope

[doct9960]

In the Moss book, torsional moment was not included in the design of a Type 3 trunnion (no lug). I think torsional moment due to friction should not be neglected.

For those who design lifting trunnions, what coefficient of friction do you normally use in calculations?

I don't design lifting trunnions. I just like to have a "feel" on how big the trunnions and repads would be for a tower I'm currently working on.

 

[bernoullies123]

I haven't seen any designs where they have considered torsional stresses, but it not to say they are not existent. Most lifts are I've done, the slings are generally lubricated, but some have also dogged into the paint also. You can always determine the torsional load produced based on steel on steel factors like 0.15 to 0.25 given in Moss page 61 and check those against the maximum bending loads produced in the horizontal and 90 degree angles of the lift with you dynamic load factors applied etc. to see how negligible (or not) they are.

Let us know what you come up with.

 

[doct9960]

Even if the sling is lubricated, there is a possibility it would stick or "bite" on the end plate or cable guide.

I guess the torsional load is insignificant compared to the bending loads. I may still need to determine the torsional load to check the weld sizes. I'll try a CoF of 0.15.

The tower is 85 m [279 ft] high x 5.5 m [18 ft] diameter, with an erection weight of 650 tonnes [1,433 kips]. Trunnions would be attached to a 19 mm [3/4"] thk shell. Distance of trunnions from COG is 27 m [89 ft]. COG is 42 m [138 ft] from base.

I have not ran the numbers yet, but I have a hunch the local stresses on the shell will determine the size of the trunnion.

 

[chicopee]

If the tower is going to be erected with a two crane pick, I would do a torsional stress analysis to complement the shear and bending stress analysis because during the lifting stage of the tower, one end will be raised by the crane while the other end will stay clear off the ground and that procedure will rotate the tower to a vertical position subjecting the truunions to torsion .

 

[doct9960]

Ran a quick calc with just the longitudinal moment (vessel in vertical position) and I'm getting excessive local stresses (WRC 537) even if I bump up the shell thickness to 38mm. The trunnion design would definitely need FEA. I'll leave this one to the experts. For sure they will consider torsional loads in the design.

I guesstimate the trunnions to be around 900mm diameter with stiffening rings at the top and bottom.

 

[chicopee]

doct9960, Have you studied the truunions shown in the video presentation shown in your previous post "heavy lifts" and how these columns rotated while being picked? Also have you examined the bottom attachments of these columns which were hooked to the smaller capacity cranes?

 

[doct9960]

chicopee,

Yes, I did study the trunnions in the video and for sure there were torsional loads on them. On the tower I'm working on, I assumed a CoF of 0.15 and used an impact factor of 1.35. The effect of the torsional moment on local stress was negligible. (bernoullies123, you're right. They are negligible.).

chicopee, by bottom attachment, do you mean the tailing lug? Anything special I need to know about them?

The tower i'm working on is very similar to the one in the video below, except below the trunnions, the tower will be "dressed" as much as possible.

http://www.heavyliftspecialist.com/...ree-crane-lift-of-a-512-tons-pressure-vessel/

 

[DSB123]

Hi There,
Just wondered where you came up with a COF for steel to steel of 0.15? That's nealy the limit value for PTFE to Stainless Steel(polished). Steel to Steel is significantly greater then PTFE to SS otherwise why use low friction bearings (PTFE/SS)?

 

[doct9960]

DSB123,

0.15 is for lubricated steel to steel. That's just my assumed value for the CoF between wire rope to steel. Some references say it is lower than that, say 0.08.

 

[chicopee]

The tailing lug is offset so that the crane hammer head will not interfere with the columns when they are nearly vertical and the bottom sling is ready to be loosened.

 

[Soln]

The lift designer might consider using a rigging block, or snatch block to avoid torsional effects in the trunnion.