Build Your Own Weak-link Tester

A weaklink is an absolutely essential element for the safe operation of any towing system and must be used for all tows to ensure the safety of the towing operation. It is useless to use a weaklink if you do not know the force that will cause the link to break. Tying random lengths of kite line, worn out tow line, or shoelaces together to make a weaklink is useless unless you know how much force needs to be applied to break the link. Using calibrated weak links provided by a manufacturer like TowMeUp.com doesn't give you a true picture, unless you use the exact same link or line material that was used in the test fixture to calibrate the weaklink. Fortunately there are easy ways to conduct your own experiments to determine the breaking strength of the weak link.

You could use a load cell with a strain gauge tester like we do, but that's a bit out of the normal tow operators budget. You can hang a pail on the end of a weak link and add weights to the pail until the line breaks and then weigh the weights, but you end up with broken toes. Or, you could be a simple pneumatically powered tester like the one shown above. We built this one for around $100 using surplus materials in our shop. You should be able to obtain the components locally or via mail order from many surplus or pneumatic suppliers. Here's what you need:

- A base to mount the components on. Ours is a hunk of 6061T6 Aluminum, but even a stiff board will work just fine.

- A glycerin filled maximum force recording pressure gauge in a range appropriate for the cylinder and pressure used. Ours goes from 0 - 200 PSI.

- A pneumatic cylinder of an appropriate size. The larger the bore, the larger the force you can apply. A 2" or 3" cylinder with a 6-8" stroke will work fine.

- A pressure regulator to limit the input air pressure from your air supply (this is optional).

- A 3/8" ball valve to allow air to flow to or from the cylinder.

- A couple push button air valves, Miscellaneous hoses and couplers, fittings, and hardware.

Operation is fairly simple. The cylinder size used and the pressure applied to it determines how much force can be exerted on the weaklink. In this case the cylinder used is a 2" cylinder with a 0.625" rod extending from it. If you supply air pressurized to 100 pounds per square inch to the cylinder to extend the piston rod you can generate a force proportional to the surface area of the piston that the air acts upon. If you are extending the cylinder the 100 PSI air pushes on a piston surface area of ~ 3.1415 square inches, and can exert a pushing force on the piston rod of 314.15 pounds. We can't use this force very well, since we really need to stretch the weaklink to break it, so we need to figure out how much force we can apply by retracting the piston rod, and then hook one end of the weak-link to this rod.

The force is computed the same way except that we have to subtract the surface area of the piston rod from the area of the piston itself , since air can't affect that area. This gives us an effective surface area of ~ 2.85 square inches. If we applied the same 100 PSI to the cylinder to retract the piston rod we could pull on our weak-link with 285 pounds of force.

By using a maximum reading gauge (this is a special gauge with 2 pointers) installed on a T fitting on the input side to the cylinder that will allow the piston to retract, we can monitor the pressure inside the cylinder as the piston rod retracts. We can watch the pressure rise on the gauge and the rising needle indicating the pressure pushes another recording needle along with it. We use a glycerin filled gauge to prevent the needle from bouncing and giving erroneous readings. Use of this device is straightforward. You need to apply pressure to extend the cylinder all the way. Attach a link similar to the one on the end of your leader line to the end of the piston rod. Attach one end of your weak-link to this link. Attach the other end of the weak-link to a connection similar to the type on the tow bridle used and make sure this is well fastened to the test fixture. Zero out the indicator on the pressure gauge. Set the inlet air pressure regulator (if used) to a value that will yield a force appropriate for the link being tested. Slowly open the valve to supply air to the cylinder allowing it to retract. As the link stretches, the pressure will rise, and continue rising until the link breaks. As soon as the link breaks the pressure will fall off until the piston rod retracts all the way, and then it will climb to it's maximum value. You need to either turn off the air supply, or push a valve to extend the cylinder before the piston rod retracts all the way and the increasing pressure causes the recording needle to rise higher than at the point the weak link broke, to keep from getting a falsely high breaking strength. Take a look at the test example shown below:

You should break enough samples to ensure consistent results. Typically a sewn link should always break at the end of the stitching, and a knotted link should break at the end of the knot. If not, you line material should be considered suspect.

Use common sense in the construction and use of this device, and ensure eye protection and other safety gear as appropriate is worn during operation.

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