Modern pulleys

Why would anyone wish to write, let alone read, an article on the subject of pulleys? They are after all just little wheels mounted low down in the base of bellframes to smooth the passage of the bellrope through the hole in the belfry floor and to accommodate the change in the position of the rope as the bell passes from handstroke to backstroke and vice versa. That's all there is to them isn't it?

Well no, not really.

These small, largely unseen and frequently forgotten components contribute more to the "go" of a bell than any other item of comparable size or cost. Indeed it is no exaggeration to claim that the performance of pulleys and the "go" of the bells are directly linked.

Pulleys started life as rollers usually around 3" in diameter and some 8" to 10" long, but by the mid-18th Century the design had changed to the narrow wheels that we see to this day. Although this change in shape took place over 200 years ago the term "rope rollers" persists to this day. Through the 18th, 19th and early 20th Centuries the design, and often the manufacture, was fairly basic with drive fit iron spindles rotating in the sides of the pulley boxes often with V.shaped notches being cut just above the spindles to ease the introduction of lubricant.

With the introduction of ball bearings to replace main bearings at the headstock ends, the Whitechapel Bell Foundry simultaneously fitted ball bearings to their pulleys as the company fully appreciated that reducing the coefficient of friction in the main bearings would serve little good purpose if the coefficient of friction in the pulley bearings was not similarly reduced.

Friction of course is only part of the story. It takes approximately two seconds for a bell to rotate through a full circle. At the beginning and the end of this cycle the pulley is stationary. As the bell passes bottom dead centre the bell and pulley attain maximum speed with the pulley rotating some 10 - 15 times faster than the bell depending upon the relative sizes of the pulley and wheel. The acceleration of the pulley therefore during the first second is enormous as it is in each successive second as the pulley is decelerated to zero then accelerated again and so on. Regardless of the ease with which the pulley may turn, its inertia must be overcome by energy provided by the bell ringer during each and every second of ringing. It must follow therefore that the ideal pulley must exhibit both zero friction and zero inertia.

Conscious of this double requirement, Whitechapel attempted to minimise inertia from the outset by selecting relatively lightweight hardwoods from which to manufacture the pulley sheaves, and subsequently by machining recesses in the sides of the sheaves to reduce their mass.

During the 1930s a number of austerity measures were introduced by the company to reduce costs generally and these included replacing ball bearings in pulleys with oil impregnated bronze bushes. Although these bushes proved satisfactory when new, their performance deteriorated rapidly. If the bushes became clogged with grease the pulleys rotated very stiffly indeed whilst even the slightest wear in the bush or spindle was accompanied by an unpleasant rumbling sound which could only be overcome by the addition of more grease.

Whilst the evidence for the reintroduction of ball bearings was overwhelming, the company was aware that slight wear in the ball bearing unit was accompanied by disproportionate sideways wobble in the pulley circumference; a characteristic which did not occur with plain bearings. Whitechapel therefore pioneered the design of a ball bearing assembly wherein two single row deep groove ball bearings were fitted in to a machined steel housing which in turn was a drive fit within the centre of the pulley wheel. The two bearings were placed approximately 1" apart with their outer faces aligning with the outer faces of the pulley wheel. In this way a combination of low friction and lateral stability was achieved. As with the two previous designs, lubricators were fitted to the spindles but in this case grease was introduced into the space between the two bearings. The new design was considerably more costly to produce than the previous two, however the company felt the expense worthwhile as it was clear from the outset that the new pulley was vastly superior to either of its predecessors. Indeed, so pleased was the company with the new design that it believed at the time it had finally produced the perfect pulley.

Initial optimism however was short lived for within six years three possible faults had come to light.

Firstly, the design incorporated no feature which restricted the quantity of grease that could be introduced via the lubricator. Over greasing was therefore possible which in turn caused the pulleys to rotate stiffly.

Secondly, a small number of pulleys warped or split within a few years of installation which created circumstances in which a worn bellrope could jam between the pulley wheel and the inside of the pulley box thus bringing the ringing of that bell to an immediate halt.

Thirdly, the pulleys continued to exhibit the feature of all timber pulleys in that they wear out of round. That is to say that the wear of the pulley wheel along the grain of the timber is faster than the wear across the grain. As the pulley wears so it becomes increasingly oval; a feature that can be felt eventually by the bell ringer.

In addition to these three problems, the company was aware that little had been changed to reduce the mass of the rotating pulley wheel.

So it was in 1966 the company yet again embarked on the task of redesigning its pulleys with particular reference to the four issues listed above. The double ball bearing assembly had worked well and was retained. To avoid over greasing, pre-lubricated and sealed for life bearings were introduced whilst the spindle lubricators were omitted. Timber however was replaced with polypropylene. It was in 1963 that the company had successfully introduced this material for chime pulleys and so in 1966 a prototype was manufactured with a larger pulley suitable for a change ringing bell. This prototype was put on test in a local tower where it continues to perform well to this day.

As a replacement for timber, polypropylene neither warped nor split, nor did it wear out of round. Additionally, the design of the pulley wheel produced a 40% weight saving when compared with its timber equivalent. Yet again the company believed it had produced the perfect pulley but again initial optimism was short lived.

In 1970 the company discovered that if a pulley became jammed solid as a result of material wedged between the pulley wheel and the pulley box the bellrope would wear a groove in the surface of the static pulley at a faster than expected rate. Having taken specialist advice, the company changed the material from polypropylene to nylon, and, on the basis of the greater strength of this new material further reduced its weight. The combination of an excellent bearing and spindle design coupled with a lightweight pulley wheel produced the best performance yet and through the 1970s and 1980s the company was increasingly complimented by clients on the "go" on the bell it had hung.

Initial optimism concerning the "perfect pulley" however was relatively short lived. By 1985 it was apparent that whereas most of the nylon pulley wheels both performed and lasted well, a small number were wearing at an unacceptably fast rate. Detailed discussions took place between the company and its supplier concerning the consistency of quality of the pulley wheel mouldings until in the 1990s Whitechapel took the decision to again change the material and on this occasion to change the supplier also.

Whitechapel's current generation of pulley sheaves are machined from cast nylon bar and although there are differences in detail between these machined pulley wheels and the previous moulded pulley wheels they are fully interchangeable. The density of cast nylon is greater than that of either moulded nylon or moulded polypropylene and whilst the latest design pulley wheel exhibits excellent wear characteristics the mass of the pulley is now greater than the previous designs although still significantly lighter than the timber sheaves that were employed until the mid 1960s. Mindful of the increased strength and durability of cast nylon Whitechapel have refined the design further to reduce the mass of the pulley wheel whilst maintaining its inherent durability. Again, the latest design is interchangeable with previous designs.

Whitechapel's search for the perfect pulley therefore continues. The latest design has neither zero friction, nor zero mass, nor indeed infinite life. The strides made by the company towards these goals have been enormous. So next time you have concerns about the "go" of the bells at your local tower don't overlook the pulleys. They are more important than you may have thought.