FEATURE CLUTCHES, BRAKES & COUPLINGS L
ocated in New Hampshire, the Mount Washington Cog Railway transports visitors to the highest peak in the Northeastern US.
In operation since 1869, it takes travellers on a six mile, three hour, round trip to a height of 6,288ft. above sea level – with an average grade of 25%, although some sections approach nearly 38%. As such, the train experiences extreme weather conditions, from wind chill temperatures down to -102˚F to very high winds (a measured surface wind speed of 231mph is a Northern and Western Hemisphere record). The fleet currently consists of a pair of steam locomotives, seven biodiesel locomotives and seven passenger coaches. The first diesel-powered locomotive was
designed primarily by Al LaPrade (who is now serving as a consulting mechanical engineer) and was built on-site by the Cog’s shop crew. In 2001, LaPrade also designed the sprag
clutch system for the passenger coaches. The original ratchet-pawl system was not only noisy but could not be trusted as an emergency stop mechanism because, if it were suddenly applied with any speed, it would either bend the pawl or break a crossmember, a bearing, or an axle shaft. To meet the challenging backstopping
requirements, Formsprag FSO clutches have been installed on every coach. During the early phases of the sprag clutch
system development, LaPrade visited the Bretton Woods ski area to view a Formsprag clutch that was installed on a chairlift carousel. The ski lift clutch turned out to be a similar model to the FSO unit specified for the railway coach retrofits. The specific Formsprag Model FSO 800 clutches,
featuring PCE sprags with Formchrome and Formsprag ‘Free-action’ retainers, were chosen based on superior 13,000lb.ft. (17,680Nm) torque capacity and the general ruggedness of their construction. The compact 10in diameter FSO units also fit within the existing hardware and the small space available on the older coaches. The FSO units are grease-lubricated with
lip seals, specified because of the concern of lubricating oil leaking onto the adjacent caliper brake rotor which could allow the brake calipers to slip. In addition, grease will seep out slower and presumably more visibly than oil, allowing more time to identify a leak. “The FSO clutch has a lip seal mainly for
the slow application shaft speeds (~50rpm). Our maximum design forward/reverse speed is just shy of 5mph and this is a direct-acting clutch,” said Caleb Gross, mechanical engineer at the railway. “The simplicity of the lip seal allows for easy, inexpensive, service if replacement is ever needed.” A critically important FSO backstopping sprag
clutch is installed on the down-mountain axle of each coach. The sprag clutch is engaged by the brakeman for the up-mountain portion of the trip, preventing any roll-back of the passenger coach. There are 12 hydraulic calipers that hold
the clutch rotor stationary when applied. To engage the clutch, the brakeman flips a valve which applies hydraulic pressure to the brake calipers, preventing the sprag rotor from rotating. The settings are left in this configuration (only forward motion allowed) until right before the train leaves the summit.
For the down-mountain leg of the trip, the
clutch is disengaged to allow downward movement. When the engineer is in the cab and ready to go, he gives the brakeman a signal to flip the valve back and release hydraulic pressure on the clutch rotor brake calipers. The brakeman stands ready at the brake wheel on the down-mountain end of the coach for the entire trip from the summit. The locomotive stays behind (down mountain
of) the coach at all times and the two are not coupled (connected). There is an individual manual brake for each (up mountain and down mountain) axle for redundancy. Each shaft also has at least one brake drum or rotor, depending on the design. Newer air ride coaches have three modern air brakes which are all simultaneously applied through a parking brake valve. In both designs, the sprag clutch is also an added redundancy that can be quickly applied if needed to stop the coach from rolling downhill. The coach clutches are frequently field-tested.
Every time a train comes to a switch, the locomotive must stop until the brakeman signals an ‘all clear’ to proceed. Each time, the coach remains in position and does not follow the locomotive backwards downhill. Gross commented: “The first clutch was
installed on one of our coaches for the 2001 summer season. That coach has been in service since and the clutch has yet to give us a reason to consider replacement.”
Altra Industrial Motion
www.altramotion.com
baCkstopping demands
ClutCh solution meets Challenging
4 DESIGN SOLUTIONS OCTOBER 2021 2
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