Adhesive bonding lengths of fabric strapping

[duncan webster 1Participant @duncanwebster1]

<p style=”text-align: left;”>Because you don’t get equal distribution of the load in the fibres.</p>

[Bo’sun]

On Bo’sun Said:

Good morning Nigel & Dave.  Just curious, but why does a knot weaken the strength of the parent rope?

1. Tension.    When a rope is bent, the outer side is stretched whilst the inner side is compressed.   The outer bend, in tension, carries most of the load, whilst the compression side is mostly idle.
2. Leverage.   When a rope is bent back on itself to form a knot, it behaves like a crowbar, multiplying the force applied at the bends.

Both weaken the rope by concentrating forces – the load isn’t distributed equally.

 

The form of a knot determines how stressed the rope is, so some knots weaken rope more than others.  Also, in applications like lifts where knots are forbidden, designers are very careful about passing ropes over pulleys because small diameter pulleys stress and weaken rope much faster than large diameter pulleys.

Many serious accidents winding coal and men from collieries during the 19th century due to not appreciating just how quickly wire rope wound over a pulley onto a drum was weakened compared with doing straight pulls.  Straight pull tests suggested Victorian wire rope would last decades but, in practice, a few years in a colliery broke some quickly.    Cure was to: increase pulley and drum diameters; determine formula indicating life of rope as modified by bending, and changing the rope before life expired; and – very important – regular close inspection of the rope – any sign of damage meant it was due for replacement.

Damaged ropes are unpredictable, all bets are off!

Dave

 

 

[Bo’sunParticipant @bosun58570]

Thanks Gents, that makes perfect sense.

[John Haine]

On John Haine Said:

The reason I suggested ropes and knots is because for example a bowline in a reasonable diameter rope will easily hold the forces from a moored boat.  Sailors have spent centuries developing knots that hold strongly and don’t excessively weaken the ropes they are tied in.  Mountaineers trust their lives to knots on safety ropes.  Why ignore all that practical experience?

Nothing wrong with  appropriately applied knots.  One criteria is “fit for purpose”, which is why climbers avoid reef-knots.   Other knots are disqualified because they upset easily or are difficult to undo.  Not rocket science, easily looked up!

Strength-wise, knots require the user to at least double the rating of the rope.  Also straightforward, just allow for the weight being lifted.

For convenience all my infrequent lifts rely on knots.  Not a problem because I overrate the rope and check for damage.  If I did a lot of heavy machine lifting though, I’d look to straps and chains.

My climbing relatives are very fussy about ropes and knots!

Dave

[duncan webster 1Participant @duncanwebster1]

When wire ropes start to give up you get odd broken strands proud of the surface. The inspector will hold a rag loosely round the rope as it is run round.if the rag is snatched out of his hand it’s new rope time.

The ropes on the San Francisco cable cars are very long indeed. They stretch in use,  and there is a compensator in the power house. When all the adjustment is used up, if the rope is still serviceable they cut a piece out and resplice it. The guide told us they have a team full time on rope maintainance. Splicing textile rope is hard enough those guys have my respect.

 

[Nigel Graham 2Participant @nigelgraham2]

Further to Dave’s explanation, presumably different knots distort and compress the rope in different ways, so producing their own stress-points.

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