This is not a completely new concept. Spool thimbles have long been used on multi-pulley block beckets to increase the pin diameter to better suit typical sized polyester rope. But Dyneema puts a new spin on it. The rope is no longer as wide as the pin, and off center loads weaken shackles and pins, and the block or assembly may be pulled slightly crooked. If there are any sharp edges, the rope will not just contact them but be forced against them by the off-center pull.
A thimble could solve the pin diameter problem, but only if it is a sturdy tube thimble or sailmaker’s thimble that does not crush under load (light duty wire rope thimbles collapse). It won’t center the load on the pin; the assembly will be weakened by uneven loading and be cocked at an angle. The thimble will protect the rope from pin chafe, but only to an extent from the side. Wire rope thimbles (the most common sort) can also shift in the eye, allowing the sharp ends to attack the rope. Finally, they must be tightly seized to the eye so they don’t shift.
ENTER DYNEEMA
Wheel thimbles for Dyneema have unique features that solve these problems. They are the full width of the pin with a distinct center groove, spreading and centering the load. The groove is deep, both to protect the sides of the eye from chafe and to help retain the eye in the thimble without the need for a tight seizing. They don’t distort or collapse under extreme load.
The one downside is that they cannot be fitted to eye straps, pad eyes, or the bow end of a shackle because of the curvature. They only fit straight pins. Conventional thimbles are still used in those situations.
Commercial. Wheel Thimble, Blue Wave. Made from stainless steel, they are available in sizes from 5 to 8 mm. Very limited US distribution, about $25-35.
DIY Material. Can be made from aluminum, Delrin, or Nylon. An aluminum wheel thimble will be many times stronger than the Dyneema rope. In most applications, a Delrin or nylon wheel thimble will have the same or greater strength and working load than the Dyneema rope attached to it. We recommend Delrin (acetal) over Nylon, because Nylon is more prone to distortion during turning, and when wet it absorbs water, swells, and losses some strength, though it is slightly less vulnerable to UV. Plastics are also inherently non-chafing.
Fabrication. They can be turned from round stock on a metal lathe, using a tool ground to the radius of the line to make the groove. Turn the major diameter, turn the groove, bore
the center to match the pin, and cut to width. They can also be turned from blanks cut from sheet stock with a hole saw, mounting the disks on a mandrel (1/4-inch bolt with nuts and washers) on a lathe or drill press. On a drill press the groove can be made with a rat tail file. Takes a little longer, but it’s simple enough. A drill press/lathe conversion kit is recommended (Shop Fox Drill Press Lathe Attachment 4088, $50).
Strength. The failure point is compression under the Dyneema. We tested wheel thimbles made from HDPE, Nylon, and Delrin, using ¼-inch Dyneema and 5/16-inch pins. The
thinner the wheel is between the bottom of the groove and the pin, the greater the force per area and the less material to support the load. With aluminum, this can be as little as 1/8-inch (1/2- to 5/8-inch core diameter), but with plastics a minimum core diameter of ¾-inch is recommended. In general, the failure load will increase with larger sizes with the square of the total size increase.
| Thimble Material Tested | Failure Load (lbs.) | Working Load (lbs. |
|---|---|---|
| (Using 1/4-in. Dyneema) | ||
| HDPE | 800 | 200 |
| Delrin | 2,650 | 660 |
| Nylon | 2,800 | 700 |
| Aluminum (6061) | >10,000 (est.) | >2600 (est.) |
3D printing is another method. Nylon is available as filament, but a heated machine is required. The strength will also be less (20-50%) because of voids and imperfect bonding; the difference will be process-specific and we did not test this. This would take plastics out of the running for any substantial loads, even my shroud tensioners. If you try it, report back!
SUMMARY
Unlike a common wire rope thimble, a wheel thimble can’t shift and damage the line. It centers the load, keeps the tackle straight, I don’t have to seize it tightly in place, and that it protects the sides of the rope from sharp side plates. Aluminum would be the most durable material, ideal for high loads and critical applications, such as standing rigging, but plastics are suitable for lower loads and non-critical applications, while having the inherent advantage of being non-chafing and low friction. The whole reason we investigated this was a chafe problem from side plates that had been an issue, even with conventional thimbles. Not anymore.
The Blue Wave wheel thimble looks suspiciously like a low friction ring. I wonder if that would work for those of us without metal working lathes.
a. An LFR does not distribute the force across the axle. Not a major problem.
b. An LFR could shift or roll to one side. Potential problem in some applications.
c. An LFR does not have the same deep, narrow groove, and the hole through the center is not straight.
But yes, maybe. Just not optimum and probably no better than a conventional sailmaker’s thimble.
A lathe is not required, though a drill press is very handy. Cut a disks from a sheet of plastic of the correct thickness (hole saw or saber saw), mount it on an arbor made from a 1/4-inch bolt, washers, and nuts, and create the groove. Delrin will be perfectly adequate for most applications. Aluminum can be turned by hand with a light touch and a form tool ground to the correct angle. In fact, hand tools are commonly used on metal lathes for certain details the mechanical leads don’t do well. It works best with special narrow chisels, sometimes graver’s chisels, made for this purpose. Files can also work, though not for a groove this deep.
In fact, a drill press can be fitted with a simple lathe attachment (DIY or $45 from Shop Fox) that allows turning of pulleys, tool handles, and similar small items. I wrote that up too, and hopefully it will appear here soon. I have made some small things, including knobs, soft shackle dog bones, and pulleys that way. A big lathe can be cumbersome for small things. Yes, there is a learning curve, but it is easier than, say, welding.
I feel I should add a warning that a drill press is NOT a lathe or milling machine. While these methods can work for a few light-duty projects in wood or plastics, they are not constructed to withstand high, sustained lateral loads. A few plastic pulleys or wooden tool handles, OK. A small sanding drum or wire brush, fine. Machining of hard materials, no. They are built for drilling.