Crane Time: The Part With No Grease Fitting

Hygroscopic degradation of cast polyamide sheave bodies in mobile crane boom nose assemblies

The boom nose of a hydraulic mobile crane, called the boom head by some manufacturers and referred to both ways below, carries a set of polymer sheaves that bear load on every pick and wear continuously in service. Across the industry these components are described with a single word.

The nylon sheave.

That description is where the imprecision begins. Nylon is not a material. It is a family of polyamides, and its members differ enough in production method, filler content, and mechanical response that two visually similar sheaves may carry meaningfully different strength values. A part number does not resolve the question either. Part numbers identify a component by fit and function, and composition is not among the things a catalog number is obliged to convey; two sheaves can interchange on the same axle, satisfy the same number, and still be molded from different polymers.

The part itself does.

2.1 Reading the mold

Cast polyamide is polymerized inside a mold rather than machined out of extruded stock (Licharz GmbH, n.d.-a), so whatever the mold carries is reproduced in relief on every part that comes out of it. That relief is not a sticker, a paint stencil, or a stamp applied after the fact. It is part of the casting, and it outlives the paperwork that came in the box with it.

The marks worth looking for are a brand name in raised relief and, next to it, cartouches carrying a material designation, a casting date, a batch identifier, and an internal part reference. Not every casting carries all four, and the arrangement varies from one manufacturer to the next, so the absence of a given mark proves nothing on its own. The material designation is the one that settles the question, because it names the polymer rather than the product line.

One such brand is LiNNOTAM, the cast polyamide line of Licharz GmbH of Buchholz, Germany. Like most names of its kind it covers a range of grades rather than one material, running from unfilled PA 6 C through filled and lubricated variants to longer chain polyamides (Licharz GmbH, n.d.-b). A brand name molded into a part narrows the material without identifying it; the designation cartouche is what finishes the job.

2.2 Why the designation matters

Licharz designates LiNNOTAM as PA 6 C. The C denotes cast polyamide. Rather than machining a component from extruded stock, the manufacturer polymerizes the monomer directly inside the mold cavity. The result is a partially crystalline structure whose high crystallite content yields very high abrasion and wear resistance, hardness, and rigidity, and pulleys are among the applications listed for it (Licharz GmbH, n.d.-a).

Color carries some of the remaining information, although it carries it in one direction only. Molybdenum disulfide is a black solid, and cast polyamide filled with it comes out dark across the industry, whatever the supplier (Licharz GmbH, 2020b; Mitsubishi Chemical Advanced Materials, n.d.). A natural part, which presents as off-white or ivory, therefore carries no MoS₂ filler. The inference does not run backward, because unfilled cast polyamide is produced in black as well as natural (Licharz GmbH, n.d.-a). A dark sheave has to be settled at the designation mark.

3.1 The manufacturer’s two columns

The data sheet for the material publishes two sets of numbers. One set is measured on material that is dry. The other is measured on that same material after it has taken on moisture out of the air around it (Licharz GmbH, 2020a). Nothing else changes between the two columns. Same polymer, same casting process, same batch.

Table 1 lists five of those numbers, with a plain reading of each one underneath it. The first four all describe how well the material resists being pushed out of shape, and on all four a bigger number means a tougher part. The last one works the other way around. It describes how hard the material grips a steel surface sliding across it, so a bigger number there means more grip, more drag, and more heat.

Table 1. LiNNOTAM mechanical properties, dry and moisture conditioned states (Licharz GmbH, 2020a). MPa is megapascals, a standard unit of stress; the friction figure is a ratio and carries no unit.

Property	Dry	Humid
Flexural strengthHow much load it takes before it bends out of shape	140 MPa	60 MPa
Yield stressHow much load it takes before it stops springing back	80 MPa	60 MPa
Modulus of elasticity, bendingHow stiff it is; how much it flexes under a given load	3400 MPa	2000 MPa
Ball indentation hardnessHow easily the surface dents when something presses into it	160 MPa	125 MPa
Coefficient of sliding friction vs. steelHow much it grips steel instead of letting it slide	0.36	0.42

The top row is the one to carry out of here. Moisture takes flexural strength from 140 down to 60, a loss of 57 percent, and none of that loss comes from use (Licharz GmbH, 2020a). No cycles were run. No overload was applied. Nobody abused the part. The material sat in humid air and gave up more than half of what it started with.

3.2 Friction moves the other direction

Note the final row of Table 1. While every strength and stiffness value declines with moisture uptake, the coefficient of sliding friction against steel increases from 0.36 to 0.42 (Licharz GmbH, 2020a). Moisture conditioned material is simultaneously softer and less slippery against a steel rope bearing on it. The two effects compound rather than offset.

Published absorption figures for the material run to 2.2 percent in normal climate and 6.5 percent to saturation in water, reported alongside the strength values on the same data sheet (Licharz GmbH, 2020a). Moisture uptake and property loss are two measurements of one process. That pairing is rarely carried forward into how polymer sheaves are described, discussed, or evaluated in the lifting industry.

Equipment literature reflects the same imprecision found in shop conversation. The product guide for one truck-mounted telescopic crane describes the boom nose in a single line: five nylatron sheaves, mounted on heavy duty tapered roller bearings with removable pin-type rope guards (Manitowoc Cranes, 2010). Nylatron is a trade name of Mitsubishi Chemical Advanced Materials covering a family of filled nylons, and its GSM grade is cast polyamide 6 filled with molybdenum disulfide, supplied in black to gray (Mitsubishi Chemical Advanced Materials, n.d.). Where a supplied component carries a different designation and arrives in natural ivory, the literature names a filled material and the part is an unfilled one. Those are two different materials with two different property sets.

Other manufacturers underspecify in the other direction. The published technical data for one all terrain crane describes its boom head as seven nylon sheaves of a stated root diameter (Link-Belt Construction Equipment, 2024). That names the polymer family and nothing inside it, which leaves open every variable this article has covered: cast or extruded, filled or unfilled, and where in its moisture range the part is operating.

The practical consequence is straightforward. Moisture content in an installed polymer component is not observable by eye, cannot be measured with a tape or a gauge in the field, and produces no external symptom until the material is already operating well below its dry state properties. Whatever the correct engineering response, it begins with knowing which polymer is actually installed.

5.1 Three manufacturers, three different answers

Lubrication requirements for boom head sheaves are not consistent across the industry, and the spread is wider than a single component would lead anyone to expect. The operating and maintenance manual for one telescopic crawler crane directs the user to grease the boom head sheaves at regular intervals according to the machine’s maintenance schedule, listing them alongside the bearing pads and the boom hoist cylinder pins, and its lubrication table calls for filling them until new grease is visible (Tadano Mantis Corporation, 2013).

The lubrication chart for one rough terrain crane is more specific still. It lists boom head sheaves as a numbered chart item at every 50 hours, the same cycle it assigns to the fly sheaves, the auxiliary lifting sheave, and the boom extend and retract sheaves (Link-Belt Construction Equipment, n.d.).

Another manufacturer specifies the opposite across a line of lattice crawler cranes. Its published lubrication guide states that the boom and jib sheaves are “packed with grease at assembly and do not require greasing at regular intervals,” directing the user instead to inspect them yearly or every 2000 hours of crane operation and to overhaul and repack them if needed (Manitowoc Cranes, 2021). The same guide adds that where sheaves are equipped with sealed bearings and grease fittings, they may be greased at an interval the owner or user establishes.

For one component in one location on the machine, published practice therefore runs from every 50 hours to do not grease it at all. Two manuals put the boom head sheaves on a schedule with the rest of the boom’s lubrication points. The third states that they are packed at assembly and sends the user to an inspection instead of a grease gun.

5.2 Where the fittings are

A sheave grease fitting is not where a walkaround would look for it. Where a sheave carries one, it is usually reached through a hole in the rope groove rather than from any exposed face of the sheave body. A man who does not know that is looking at a sheave he has already decided has no fitting.

On most larger cranes the sheave bearing is pressed, and all of the grease is for that bearing. Nothing on the polymer body itself takes lubricant. Whether the bearing can be reached without pulling the nose sheave pin varies by design; where it cannot, servicing it means breaking down the sheave pin assembly, which is a teardown rather than a stop on a grease route.

This is where the two halves of the subject meet. All of the grease serves steel. The property established in Section 3, a flexural strength that moves by more than half with ambient humidity, belongs to the polymer, and no lubrication interval addresses it (Licharz GmbH, 2020a). The bearing has a service point. The polymer around it does not.