Hobbs or GRCS

[Rupe]

A 33kg log, dropped approx 500mm, generated a peak load of 245kg on the rigging anchor point.

 

If I have any really big snatching to do, it will be with Reg's two pulley system, otherwise I shall make more use of vertical speedline or other non shock loading techniques.

 

Is that with the rigging line held tight?? If so then of course you are going to get those loads. A better demo would then have been to see how low you can get the peak force by letting it run but still bringing it to a stop.

[Paul Smith]

245kg is well within the rope or blocks SWL.

[Peter]

Is that with the rigging line held tight?? If so then of course you are going to get those loads. A better demo would then have been to see how low you can get the peak force by letting it run but still bringing it to a stop.

 

Yes, unfortunately the emergency load cell we had didnt permit the inclusion of a rigging block, so we were rigging off a shackle, which didnt really allow for a smooth lower.

 

The demo was about worst case scenarios, can you be sure that the groundie will let it run perfectly every time, or will it get stubbed off 1 in a 100 times?

 

Worth considering imo.

 

Would be more than happy to do your scenario, maybe at another event this summer if you are up for it?

[Peter]

245kg is well within the rope or blocks SWL.

 

Yes, but how often do you snatch a 33kg log?

 

How often do you snatch a 150kg log? Multiply the loading up on that and you are looking at 1095 kg peak force.....

[Paul Smith]

Yes, but how often do you snatch a 33kg log?

 

How often do you snatch a 150kg log? Multiply the loading up on that and you are looking at 1095 kg peak force.....

 

One of the reasons I am looking at the GRCS is you wont have to snatch anything if you rig it correctly you should be able to lift and lower? For the odd times when you need to drop or even chog down you could switch to the bollard.

[Peter]

If you have a higher anchor point to crane off from, yes.

 

I'm talking about blocking down stems, which is much the same with Hobbs, GRCS, Portawrap or whatever. The only difference between the devices is that with the Hobbs and the Stein Dual its easy to pretension the lowering line.

[Paul Smith]

Peter, if you want to pre-tension the GRCS just turn the crank same as the Hobbs, not sure about the Stien.

[Peter]

I dont want to be doing any heavy rigging on the Harken winch, don't know about anyone else. If I was doing any snatching on the GRCS it would be with the bollard, which you cant pretension with the winch, cos the bollard is where the winch should be!

[Paul Smith]

I dont want to be doing any heavy rigging on the Harken winch, don't know about anyone else. If I was doing any snatching on the GRCS it would be with the bollard, which you cant pretension with the winch, cos the bollard is where the winch should be!

 

See what you mean Peter. What about if you pre- tensioned with the winch, tied it off and fitted the bollard? Little bit of a fiddle but doable?

[Rupe]

One of the reasons I am looking at the GRCS is you wont have to snatch anything if you rig it correctly you should be able to lift and lower? For the odd times when you need to drop or even chog down you could switch to the bollard.

 

Snatching is when there is nothing left to lower off. So even with a portawrap that you cant pretension, if you have a higher anchor point then you are lowering not snatching.

 

Also called negative blocking. The 245 was just an example, if you get that from 33 then waht will you get from 100 or 200kgs, thats the point Peter is making.

 

Peter, in answer to your question, can I be certian that is will work 100% of the time? Yes, with main groundie I am that confident. And if I wasn't and come up with a plan B or C.

 

I agree it is good to know the worst case scenario, and we make sure that we only push the limits when the WCS scenario is nothing disatrous. So a snapped rope over a road is never going to happen, but in a back garden when all your trying to achieve is less dents in the lawn then we push it a bit more.

 

Can't remember ever snapping a lowering line or destroying a pulley through shock loading.