You guys are hella smart so I know I will find my answer. I was looking at Fahrenheit's lift on my recent trip to Hershey and it looks like there aren't any anti-rollbacks. I see the two chains with a guard as well as a sled that lifts the train. Did the designers omit anti rollbacks since there are two chains, or am I missing something?
You are correct there are no anti-rollbacks. Supposedly each chain can hold a fully loaded train and the second chain acts as a backup should the first chain break. I also understand that the chains operate in both directions so if there is a problem the train can be lowered off the tower. I'm guessing they chose this method to avoid having to evacuate the train while it is in the vertical position.
I suppose the probability of both chains braking at the same time is the same if not lower than a mechanical rollback failure, but there is a tiny part of me that's still unnerved by the lack of a fixed rollback system on that ride.
Well correct me if i'm wrong but I believe there are brakes that engage between the turn from the station and the lift hill. So if a roll back were to occur, the train shouldn't back around the turn that quickly into the station.
Uh, I'm pretty sure you don't want to be coming down the back of that hill at speed. There's no pull-out.
I think Seifert is right though, that the chains simply act as the redundancy in case one fails. The elevator system of something like Pilgrims Plunge works on the same principle. I thought I saw what looks like hydraulic hoses going up out of the building next to Fahrenheit, so I assume there's a motor up there.
If there really are no anti-rollbacks that design gets me kind of nervous. What if something happens to the drive machinery that lets the chains run in a freefall, disconnected from the drive due to some type of mechanical failure? What happens if the power goes out? Is there some type mechanical brake that locks the drive? What if that fails? Redundancy is ok, but a positive lock is better.
There was a ski lift company called YAN that built a funicular without vehicle brakes. Most funiculars have brakes on the vehicles like elevators that grip the track if the vehicle overspeeds. This guards against cable breakage, mechanical failure, etc. The YAN funicular didn't have that because of redundancy. I bet you can guess where this story is going, a vehicle broke loose and crashed at the bottom after experiencing a roller coaster plunge.
If it's a hydraulic motor, and directly driven, I don't know that those can have catastrophic failures that would allow them to simply release in reverse. I mean, the fluid has to go somewhere, right? It's the same reason why your average auto garage car lift won't rapidly come down and crush the mechanic.
The thing I don't like is that even though there are two separate chains, they still make contact with one part of the train. If their is a failure on the back of the last car, then having two chains becomes irrelevant.
Follow the link at the very end of the first post by Coasterdom. It will take you to his site where there's a detailed explanation of the lift system. I didn't want to directly link to another site, so I posted the indirect way of getting there. It should answer many of your questions.
You know, you can link to whatever you want, as long as you're not blatantly spamming for your own site.
There can't be anti-rollbacks on Fahrenheit because it has to have a means for dropping the train off the lift backwards.
The combustion engine used for emergency operation does not have enough power to operate the lift. It cannot generate enough pressure in the hydraulic system to get a loaded train over the top. Instead, it can provide enough pilot pressure to open a valve in the drive motor and allow the lift to slowly roll backward, propelled by the weight of the train. That way they don't have to evacuate from a train sitting at 90 degrees.
--Dave Althoff, Jr.
What you descibe sounds simioar to a hyrostatic drive on a riding lawnmower.
You'd still have to release the pressure smoehow for it to freewheel.
It's the same reason why your average auto garage car lift won't rapidly come down and crush the mechanic.
Well, the hydraulic lifts in a garage have mechanical locks on the cylinders that act like anti-rollbacks. From a garage lift website: "Each jack has a 13-position, dual-stage locking mechanism which locks every 4 1/8” starting at 22 1/2” of rise."
Fahrenheit has no such protection. If the motor looses hydraulic pressure would that not allow the chain to freefall backwards?
The link that RGB provided said there is a holding mechanism.
I knew I would find my answer. Thanks!
Something to consider...
Hydraulic motors operate on fluid flow.
It could be that the hydraulic motors on Fahrenheit are designed in such a way that they can only respond to fluid flow. Without flow, the motor not only won't drive anything, it won't even turn. Hence the need for pilot pressure to get the things to run backwards at all, with flow restrictions limiting the maximum rate and thus the maximum speed for reverse operation.
I've been in the hydraulic building, and I listened carefully to Gary Chubb explaining the system to a whole crowd of people. But I still don't fully understand it. :)
(And I have never ridden it, but that's another matter entirely. The day I was there...the only time I have even seen Fahrenheit, the trains were in pieces in another part of the park.)
--Dave Althoff, Jr.
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