Rexnord chains selected for aircraft elevators

Modern warships are required to perform a variety of tasks while being affordable to purchase and operate. The most valuable surface vessels for navies are aircraft carriers and/or large amphibious operations craft. These large vessels provide a platform capable of ready adaptation to a number of potential naval roles, from war-fighting to humanitarian relief. The movement of equipment onboard these vessels is crucial to their roles.

Elevators must be available whenever circumstances require it; an unavailable elevator seriously reduces the operational capability of the ship and could cause it to be extremely vulnerable. Elevator availability can only be assured through equipment that has inherent reliability and low maintenance requirements.

MacTaggart Scott & Co Ltd of Edinburgh, Scotland, is a market leader in aircraft elevator technology, with over 40 year's experience of designing and installing elevators for the world's navies. The company also supplies elevators employed for handling personnel and food cargo and, heavy lifts with load capacities up to 500 tonnes.

Winning formula

According to the company, the differentiators that have enabled it to win contracts against fierce international competition are its innovative designs that achieve high performance and reliability with low through-life costs at an affordable price. This has been achieved by using low-risk concepts, proven components embracing commercial-off-the-shelf (COTS) principles, and an intimate appreciation of the operating environment and the requirements.

Aircraft elevators typically come in two distinct configurations, wire-driven four-point lift or chain-driven cantilevered designs. MacTaggart Scott has designed, manufactured and installed both of these configurations, but experience has demonstrated the chain-driven cantilevered design to be superior in all respects.

This is because the wire-driven, four-post lift configuration tends to be restrictive on the surrounding ship design due to the large jigger assemblies, turning sheaves and spaces required to manage the wires. In addition, through the life of the vessels, the wires are prone to stretch, and unpredictable fatigue results in extensive maintenance requirements.

Drawing upon past experience, MacTaggart Scott designed and developed a hydraulically-powered chain-driven cantilevered elevator that is simple and elegant, easy to operate, requires low maintenance and imposes the minimum of restrictions on the ship's design. The selection of modern engineered chains to provide lift enabled the use of synchronised rotary hydraulic motors to provide the motive power.

Motor-mounted sprockets

A compact, reliable synchronised hydraulic motor unit was selected for the MacTaggart Scott aircraft elevator. This has reduced the installation footprint and provided a reliable, low-maintenance motive power source. The drive unit uses two independent MacTaggart Scott 100 series hydraulic motors with integral fail-safe mechanical brakes, interconnected via a synchronising shaft. Chain drive sprockets fitted directly to the motor main shaft provide the drive to the chains. The design is compact, reliable and features multiple redundancy - all in keeping with the design philosophy.

The Rexnord chains selected for this application also fit with the design philosophy. Both duplex and triplex Rexnord chains have been successfully fitted to MacTaggart Scott aircraft elevators for over 40 years on multiple installations. The Royal Navy fleet support vessel RFA Argus has two MacTaggart Scott aircraft elevators installed, which have operated safely for over 19 years. The Rexnord chains fitted to this vessel have been continually monitored by MacTaggart Scott and show no evidence of deterioration.

Longevity is said to be ensured through the quality of chain supplied by Rexnord. Their simple maintenance requirements ensure the limited ship maintenance staff available can undertake all necessary maintenance tasks, resulting in the through-life cost of the elevator equipment being kept to an absolute minimum.

The safety inherent in the Rexnord chain also exceeds the most stringent naval rules. A single Rexnord chain can safely support and operate the MacTaggart Scott aircraft elevator, so the two fitted in typical elevator installations provide a level of safety and redundancy unmatched by wire-drive alternatives.

MacTaggart Scott is currently applying a Rexnord chain-based system to the new Royal Navy future carrier (CVF) programme, and it will be employed for the two 70-tonne SWL deck-edge aircraft elevators being fitted to these aircraft carriers. These elevators compete in size with those fitted aboard the gigantic American aircraft carriers, but will exceed their lifting capacity and have lower maintenance demands.