Safety Tether

It depends upon the "springiness" of the tether rope and its ability to sustain a jerk load as it goes tight, which is a function of the speed of the thing as well as the angle and length of the descent ramp. Generally, the shorter the rope, the better it will withstand the jerk, as the less distance it has to run, the slower it will be going.

One might be tempted to discuss the matter with a few rope manufacturers over the telephone?

In the limit, only a practical test under controlled circumstances will demonstrate the effectiveness of the solution.

The mooring rope will be 8 m long. Unfortunately the only strong anchor point is the tow ball hitch. That means the runaway will leave the ramps to travel 5.5 m. further before it's stopped by the tether. I was advised by the rope supplier to opt for nylon as it is much springier

I guess its a case of "suck it and see" with plenty of distance behind the van. Problem being I don't know how serviceable the rope will be for future use after testing.

Why 8 meter long?

Can you not attach to the front/leading end of the machine, the tether being length appropriate to allow travel to the end of the ramp only, perhaps long enough to allow departure from the ramp?

This tether seems to be part of a belt and suspenders scheme. Should the drive fail again, don't let the machine pick up too much speed.

Why not just lower the machine with a winch and eliminate this problem....?

I have a winch permanently fixed to the machine but it takes too much time to set it up and creep it up/down the ramps each time I need to up/download. I need a quick easy to remove tether.

20 years ago I drove a pick-up truck with a large generator/welder in the back.

The winch (almost the same cheap one in SE's suggestion) was mounted near the cab after attaching the single carabiner I could creep the behemoth up or down the ramps in just a minute.

It was not too slow OR too dangerous.

I can't imagine loading/unloading more quickly AND feeling safe about it.

________________________________

Speaking of welding? What's up Lyn?

The probability of another drive failure is very small since I have solved the reason for the previous drive failures. My grinder has happily been traversing ramps. This is a back up only, in the unlikely event this may happen again. This is why I do not wish to depend on a slow winch set up as I frequently need to up/download between jobs.

Sounds reasonable...if I had drive wheels on the generator/welder I myself would have thought differently.

I was generally concerned about skidding off the ramps sideways.

How about a

DitchPig Kinetic Vehicle Recovery Rope, 1/2 in. x 20 ft. (7,300 lb. Breaking Strength) $50.

https://www.tractorsupply.com/tsc/product/ditchpig-kinetic-vehicle-recovery-rope-1-2-in-x-20-ft-7-300-lb-breaking-strength

A test would seem the easiest (most entertaining) way to find out :)
Del

Ref.: above.

At a breaking point of 4 tonnes --- so I am advised --- one would think the rope to be strong enough.

I was hoping some of you mathematicians out there can put some numbers together to confirm this.

Too many variables... some ropes can stretch more than others and may degrade quickly in the sun or other exposure...the weight is only part of the equation..

https://www.wired.com/2014/07/a-rolling-object-accelerating-down-an-incline/

I Take your point with coefficients of friction on varying surfaces etc.etc. I don't need to split hairs with such preciseness over this matter. I only need a ball park figure and then double the rope breakage strength.

Well I would say then you are safe with 4 tons breaking resistance, but taking into account that the strength of the rope may degrade at an unknown rate, I would test it for twice the vehicle weight on a regular basis....The rolling grinder is not likely to exceed much more than 1 ton force, but the stopping distance varies the actual applied force considerably...If the rope stretches 1" or 3" it makes a big difference on the actual impact force, which is what we are concerned with here....the time it takes to stop...

https://www.wired.com/2014/07/how-do-you-estimate-impact-force/

Nylon rope is stretchy though...

https://www.qualitynylonrope.com/blog/everything-you-need-to-know-about-rope-and-cord-stretching-/

Adding something like a mooring spring or mooring absorber to your tether will spread the shock over a longer period. to which the line and the grinder and the anchor (ball hitch?) will be far more robust.

I like your idea. After doubling the rope strength (I'm now working my way through these responses) once I am advised of a kilo newton figure for sizing rope tensile strength. What harm can possible overkill do with the addition of a mooring spring or absorber.

Okay, there really are too many variables, but if we neglect all the things that would lead to less jerk, we can get an idea of an upper limit on what is required.

~918 kg (grinder plus fuel woodchips new bushing kit and frictionless plane sliders) slides down a frictionless plane for 1 m change in height. All the gravitational potential energy for that meter is converted to kinetic energy.

A particular 5 meter nylon tether initially would elongate 22% before snapping, but only 9% elongation might be expected after a few years of exposure to grit, moisture, UV, motor oil/gease, chafing, and occassionally touching the exhaust or hot grinding wheel or having high speed debris impinge. So that is 45 cm from no slack to catastrophic failure.

Assuming a linear spring constant k equal to the breaking force divided by max elongation prior to snapping, then

mass * 9.8 m/s² * height = k * (max elongation)²

and since k is equal to the breaking force divided by max elongation... and substituting in values....

breaking strength needs to be

>= 918 kg * 9.8 m/s² * 1 meter ÷ 0.45 meters.

So the tether needs to be able to relaibly withstand around 20000 N at most if the assumptions are true. That is pretty closr to being able to support a 2040 kg weight hanging.

It is important to remember that a little weathering, abrading, moisture, heat, and UV does more than reduce max elongation. Tensile strength is also reduced.

A few years of outdoor use may degrade nylon to 20% or 10% (maybe even less) of original strength with still giving the appearance of remaining intact.

Knots and splices and other attachments can also impair the line strength.

That could be specifying a nylon tether that could carry 20,000 kg or more when new....and that is before a safety factor.

.

That is obviously overkill. The frictionless plane alone porks the number up considerably.....but how much do the tires and ramp tread and dirt show things?

Numerous things reduce the needed strength:

• Longer tether to allow more total elongation as long as additional speed is not gained over additional tether length.
• mooring springs/absorbers...again elongation
• reducing change in height from loaded to unloaded.
• friction/drag that limits gain on speed for a certain change in height. An example would be keeping the tires at the low end of the recommended pressure range.
• Limiting max mass of tethered grinder.

It would seem from your comments that the grinder is loaded inside the van and that the tether needs to be attached to the rear stake and to the tow hitch on the rear of the van.

It would also seem that you are loading the grinder back first into the van and then driving it forwards to unload. This, combined with the tether point locations, requires a far longer tether than if you were to load the grinder nose first (draw yourself a picture). The tether could then be made just long enough to get the machine off the ramps. Using a suitable vehicle type shock absorber could be an alternative to a spring.

You didn't say what type of van it is, make, model, single, twin axle. My thoughts were that you loaded it in a particular way do to the center of gravity or,,,

It is a High top VW LT35 panel van with a side sliding door and twin rear doors whose bottom rests on a sill 650mm from the ground when loaded.

You could mount a mooring cleat or bollard forward of the grinding machine. Pass a rope two or three times around the cleat/bollard. Wearing gloves, hold on to the loose end and let the rope pay out according to the speed of the descending machine. If something breaks, friction around the cleat/bollard will enable you to keep the load into a gradual descent until on flat ground. (Like rappelling.)

So, holding onto their rope with one hand and the machine's controls with the other, whilst backing down a ramp ?

I was going to suggest he contact his carrier to see if he could get a reduction for liability by using a safety device.

I have another guy with me who could hold onto the rope while I operate the controls

You could use a simple pulley with a descender...

..."Very easy to use: the rate of descent is controlled by varying the grip on the free end of the rope"...

Indeed; that's even better. (I haven't climbed in a long time and haven't used such a

device, but it would be safer.)

This too is a worthy idea as it will mean a longer length of suitable rope with the proviso that the descender can take the appropriate weight/force of the machine should it want to break away

effective but impractical. It will mean a structural change to the van to secure the bollard which will incur type approval for insurance purposes.

How about a tow strap. They come in many different lengths, end hardware, and strengths. They have a lot of stretch, are cheap and available everywhere. We used to jerk on them as hard as we could with trucks to get others out of mud holes. Seldom broke any.

Good idea but it will be dependant on their length. If its loops were designed to be adjustable that may be a good option. Interfering with a re-loop will nullify any warranty against failure.

Chock, that!

You totally miss the point with this suggestion.

Please feel free to tell that to anyone who cares.

I would look into some type of inertia reel or centrifugal device, we had numerous types with working at height or in hazardous areas. Even the suggestion of a winch would not slow down the loading or unloading enough to overcome the concept of safety.

Sounds very effective but expensive. The machine fills up my van with about 30cm of forward room left between the bulkhead and front wheels of the machine. Attaching such a device securely will be a challenge. I am currently running at my van's load limit.