Porta-Wrap Calibration
Been scratching my head on this since a thread on porty nomenclature came up. As a device the porty is a pure example of tension ratio, big rope tension side holding the log and small tension side being held by the groundie. It's ratio is determined by the wraps and bends of the rope going through it. In the nomenclature thread I noted the similar BMS Belay Spool and how I previously measured the expected exponential ratio change with number of wraps and spotted the particular to BMS constant of x3 ratio increase per additional wrap. A porty and BMS having roughly the same drum diameter one would expect roughly the same size constant for a porty. But things differ overall. The BMS has a pure tangential rope entry and exit while the porty has a 90 degree entry bend around a smaller diameter pin and also a similar 90 degree bend exiting toward the groundie. The entry and exit have some amount of "equivalent wrap" contribution that is always present. Also, the BMS is used at an overhead rig tip in lieu of a pulley while a porty is almost always used with an overhead rig tip block/pulley. So it makes sense to include the effect of the block similarly as the 90 degree bends, as a base or lowest tension ratio case. A challenge comes as to how to measure this.
When I measured the BMS I had load cell on each end of the rope, tensioned the rope pulling from one end and watched/measured the rope take up its stretch and travel around the bollard, both rising and falling tension. I could see the simultaneously logged values and confirm that there always was a constant ratio and that it reversed with rope travel direction. Same procedure yielded a tension ratio of x1.2 for a pulley - ball bearing or bushing. Chanced across Peter Donzelli's research and discovered he was perplexed when his overhead rig tip block load cell didn't read x2 of the load on the end of the rope. In the SRT Base Tied Tip Forces thread I checked Pete's numbers and found out he had the same tension ratio 1.2 as I measured for the pulley. Happy day. Peter went on to further explore the block/rope friction, trying to interpret it as axle/bushing friction. Probably folly as bearings just don't develop that much friction but bending and deforming a rope does. In his further work online publication the raw data table wasn't available and I don't have the oomph to reverse calculate the data from the non-linear cylindrical bushing equivalence equations he used to process the data. However, he did illustrate two simpler measurement methods.
In Peter's initial work he tried for IIRC onset of motion, suspending a weight, looking for x2 tip force (which didn't happen). In his second work he had a suspended weight, overhead block and the load cell at the pulling end of the rope instead of at the x2 overhead tip. In both cases he tried for "right at the onset of motion". His interpretation of this was that there would only be static friction, no dynamic friction and no f = m x a inertial force effects. Without a high sample rate data logging and a second load cell in the system to check for force sum inequalities I think it's virtually impossible to nail that instant, procedurally or timing the data pick. IME it's even hard when you log the movement of the rope. However, a field method aiming for constant slow speed is do-able, where you acknowledge you're going to get an f = m x a dip when starting the log downwards and a corresponding bump when you halt the downwards motion - you've got several feet of travel at controlled constant velocity to see a steady reading on your eg Enforcer load cell. Bingo. As a bonus, the objective is actually the dynamic friction/losses scenario of during normal use.
From a useful numbers point of view, the two 90 deg and the one overhead x1.2 rig pulley will always be there, so the objective is to get that base combined system tension ratio. Then increment by 1 wrap and repeat. Add another wrap and repeat. If you can go 3 wraps and the groundie end hasn't gone too floppy you can check for the constant x-something (approx x3?) tension ratio change per wrap - and you're calibrated. As a side product you can also note how much force at the groundie's hand feels comfortable or uncomfortable (eg 10 lbs +/-?) to give a target for rigging operations.
And make a chart
One gotcha could be any excessive stretched out spiralling of the wraps on the porty messing with the bollard equation applicability. To be seen. Could be just a small deviation of x per wrap constant. And it's to a degree rope/porty combo specific. Also to be seen. Maybe it'll turn out to be a roughly universal calibration.
