June 2020

40 n BRAKES, CLUTCHES AND COUPLINGS June 2020 www.drivesncontrols.com Couplings give US drawbridge a lift after it fails to close T he River Grand is the longest river in the US state of Michigan and one of three major tributaries of Lake Michigan. It is a busy route for shipping, so any bridge less than 60ft (18.3m) above the water must open and close in accordance with US Coast Guard requirements. The US 31 bridge that spans the river at Grand Haven is one of the busiest in the state, with an average of 60,000 vehicles crossing it every day – rising to 120,000 during busy weekends and holidays. The double-leaf bascule bridge was built in 1958, replacing an earlier wooden swing bridge, and has been a vital link in the local transport infrastructure ever since. It has had to cope with a continuous year-on-year growth in traffic. However, in recent years, it started to experience malfunctions, culminating in the bridge becoming locked in the open position during the 2018 Memorial Day weekend. This failure caused major traffic jams, so the Michigan Department of Transportation decided it was time to take action. It considered building a new, taller bridge to replace the US 31. But preliminary investigations revealed that the space a taller bridge would require – once all of the approaches had been built – would have had a negative impact on the local community and environment. Instead, a major refurbishment of the existing bridge was commissioned, with a newmotor drive system taking priority. The project involved installing new generators, motors and control cabinets, clutches, brakes and about 21km of cabling. The 211m-long bridge was also split into four individual spans so that each leaf can now open independently, allowing traffic to continue flowing in one direction even if a leaf malfunctions in the other direction. The new drive system is expected to operate for decades to come, during which time some failures and maintenance issues will inevitably occur. So, as part of the project, the authorities decided to upgrade the bridge’s backup drives as well. The backup system is designed to engage rapidly in the event of a failure to keep the bridge working while the main drive is being repaired. Electric stationary field couplings (SFCs) have been installed on the backup drive shafts to ensure reliability even in adverse environmental conditions. These couplings – a new design supplied byWarner Electric, a brand of Altra Industrial Motion – don’t need brushes or collector rings to carry current, making them robust and suitable for harsh environments and varying temperatures. The bearing-mounted clutch couplings have static torque ratings of 949Nm and a maximum rated speed of 2,000 rpm. In-line mounting results in a compact package, while the stationary field design means that the clutch will not be affected by the varying weather conditions. The couplings are ready to operate at a moment’s notice whenever the backup drive system is needed. They are available in either flange- or bearing-mounted styles, and a splined hub-and-adapter serves as a coupling for in-line shaft applications. The refurbishment of the US 31 drawbridge has improved its reliability, while also speeding up its opening and closing times by several minutes. By Memorial Day 2019, the bridge was back in service and the Michigan Department of Transport could reassure drivers that there would be no repeat of the prolonged delays that they had experienced the year before. n When a busy drawbridge in the US failed during a holiday weekend, officials decided that it needed a new motor drive system. As part of the project, a backup drive was installed, using electric clutch couplings to ensure that traffic would keep moving, even if the primary drive failed. The bascule bridge that crosses the River Grand at Grand Haven, Michigan, has four independently operated leaves, each offset by large counterweights The electric clutch couplings on the backup drive shafts are designed to engage rapidly in the event of a failure of the main drive system.