WHILE the liftspan on the Bridgewater Bridge is getting a rest from this week, beginning March 1, it’s worth having a look at a mechanism that’s now 76 years of age.

Put another way: in human years, that’s well past retirement age.

Certainly, the steel structure is the oldest surviving vertical lift bridge in Australia. These days, the liftspan is operated by one authorised person from the Department of State Growth; a phone call from the captain of the vessel gets a lift booking made. In the most recent two years, the span has been raised around 90 times.

So how does it work?

At its heart are 8-pole, 30 kilowatt electric motors (they’re low speed, high torque) mounted on each side of the ‘operation house’ at the top of the bridge.

The motors rotate winch drums, one in each corner of the road-track section being lifted, which in turn haul the supporting steel cables up or down.

An idler sheave – much like those grooved wheels on the exterior of your car engine – positions the cables while guides on the control deck and vertically along the tower columns keep them aligned. The lift span can be lifted to a 30-metre height, but it’s been kept to around 15 metres.

The shutdown of the lift span, until November this year, means only very small vessels will be able to navigate this river section at Bridgewater.

With the roadway in place, there’s only about two metres of headroom at low tide and 1.5 metres at high tide.

Keep your head down