Sheave assemblies: A vital part of ropeways maintenance programs
A BCSA safety officer doing a periodic assessment of a fixed grip chairlift noticed that a tower’s combination sheave assembly had been set up incorrectly, posing a serious risk of deropement. In this specific case, the upper sheaves did not have the correct preload (down force) set. Sheave assemblies are mounted on top of ropeways towers to guide the haul rope and allow the carriers to travel safely up and down hill. There are three kinds of sheave assemblies:
- Support assembly - Haul rope is on top of the sheaves
- Hold down assembly - Haul rope is on the underside of the sheaves
- Combination assembly - Sheaves above and below the haul rope
Combination sheave assembly
Each ropeway manufacturer has specific instructions on how to set up their combination sheave assemblies, which may include details regarding specific spring tension, the amount of preload set on sheaves, specific measurements and clearances. These requirements must be followed or potential problems can result, such as excessive forces applied to the assembly components, premature wear of sheaves, rough ride for passengers, damage to carrier grips, or misalignment of the assembly.
A sheave assembly must also be aligned correctly to ensure that the ropeways haul rope is running in the middle of the sheaves. A misalignment can cause excessive wear and the potential for a haul rope deropement, which is when the haul rope leaves its normal running path on the sheaves and drops below or above the level of the sheaves. The most common deropement occurs outside of the sheave assembly and the haul rope is retained by the cable catcher. Sheave assemblies must be equipped with a non-resettable deropement switch that will stop the ropeway if it leaves its normal running position.
In a worst case scenario, the haul rope is not retained by the cable catcher and the haul drops below or above the sheave assembly. This can result in injuries or death to the passengers riding the ropeway and serious damage to the ropeway.
In this specific case, the safety officer found that the upper sheaves on a combination sheave assembly did not have the correct preload (down force) set and this could have led to a potential deropement of the ropeways haul rope.
BCSA has investigated incidents where the incorrect set up of a combination sheave has resulted serious injury and components of a sheave falling to the ground.
Components of a sheave assembly
Understanding how sheave assemblies are installed and set up is a vital part of a ropeway contractors’ maintenance program. This information can be found in manufacturer’s manuals, safety bulletins issued by the manufacturer, safety orders issued by BCSA, and onsite training provided to your maintenance staff.
It is the responsibility of the ropeways owner to ensure that all ropeway components are being maintained by “trained and competent persons” (see CSA Z98-14 clause 12.2.3).