Need speed switch for DIY funicular/hillside elevator

Excuse me??? I resemble that remark!!!

The current set up is SOOO much better than a motorized winch. I haven't gone into much detail about it, but is being driven by a 5 HP 3 phase VFD (variable frequency drive). It is driven by a $1,500 motor controller that is programmable to vary the ramp up and ramp down speeds as well as taking switch (wired and remote) inputs to trigger events. The whole setup was designed for over 5,000 lbs (with the exception of the rails and cart) and should never have a failure event, but I'm not so naive to assume it would never happen.

We are just trying to eliminate all possible forseen sources of failure, and the most catastrophic I can think of besides the cart falling over is a main shaft break causing a freespool. The drums are secured with 3 bearings on solid steel pillow blocks, so I don't think if the shaft breaks the drums would fall off. Of course another source of failure would be a cable break, so we had 2 setup instead of just one.

I don't think that all the suggestions are necessarily goldbergian. There are 2 possibilities, stop it at the head unit (at drums) or stop it at the cart. Each offers its challenges, but not impossible. Its just a balance between what I can actually build, and what will take the least amount of maintenance.

Sit there and rotate, you winch!

I hve found reading this thread most enjoyable and commend you on your work to date.

I am slightly confused as to what particular problem you are trying to solve, although I guess it all falls under the general heading of "Saving your passengers from running out of control down the hill"

I think you have to decide where the reasonable points of failure are and accept that other potential points are ruled out due to good design. This will then determine where and how you need to provide the appropriate braking mechanism.

Already you have decided that a single cable is an unacceptable single point of failure and so you have duplicated it. Given that each cable is rated at many times the actual load I would consider that 2 simultaneously broken cables to be such a small possibility that it could be safely ignored (however I have not seen such things as the attachment points at top & cart). Consider the chairlift as an example of a single cable design.

From what I have read here so far, I would think that your major area of concern is in motor or the coupling between the motor & drums. I?m also assuming that the anchoring of the motor & drums is designed to be solid under any sort of reasonable conditions, much like you don?t think your whole house is going to slide down the hill, because it has been designed no to, so doing the same with the motor would not be too much of an engineering challenge.

So in my opinion the part you are lacking is the over speed detection and braking of the drums.

I would consider solving the problem by having a third outboard disk or drum (like a car disk or drum brake) that has shoes that are held off electrically (either directly or indirectly via hydraulics) I would then pulse count the rotation of the drum to calculate the speed and in the event of over speed set the brakes. Correct operation of the pulse counting could be verified during the first couple of seconds of operation otherwise the brakes should be applied. You could probably source most of the bits using the anti locking brake system off a car.

I would also provide some form of dampening on the cable, probably in the form of a small car coil spring, rigged up in line, with the cable ends passing through it so it is working in compression. This will stop the cable from potentially whipping should the braking be a bit sudden.

Anyway, just my thoughts.

Cheers

Rohan

Going back to Doug's suggestion, you could install a pivoting arm with a pulley on the end to take up slack in the cable on the uphill side of the tram. The arm would need a hydraulic or spring-loaded strut to draw it up toward the tram. Slack feeding out beneath the tram goes up over the pulley.

The arm mechanically holds open a brake as long as it is stretched down, where it will always remain if there is tension on the cable. If the cable breaks the arm swings up, activating the brake. The can either be on a third cable or on the wheels. A cable brake should be easier to implement than wheel brakes, but the end result is the same.

The advantage to Doug's idea (as I perceive it) is that it is simple, requires no electronics and should be easy to maintain. Trial and error should quickly yield the amount of tension required to operate the arm and the brake.

Note about safety factors. When "flying" live actors or moving loads above a stage, I would select equipment rated to hold 10 times the static weight of the load. This would allow me enough margin of safety to stop a load even as it gained momentum before reaching the end of the safety cable. On a slope the load doesn't gain momentum as quickly as on a deadfall. However, depending on how quickly your safety system detects a failure and engages and the slope of the rails, the tram could accelerate, perhaps to the point of failure of the emergency braking system. Not knowing the weight of the tram, number of people who will be riding at one time, slope of the hill, etc., I can't say how strong is strong enough. It sounds as though you've given this careful consideration, but double-checking your calculations is never a bad idea.

I didn't say they were _all_ Goldbergian.

I spent about 20 years in the machine tool industry where safety was a prime concern. It was always my philosophy that safety, like quality, is something you design in from the beginning of the process rather than something you paste on at the end of the process. Another thing I strongly believe is that the possibility of catastrophic failure grows geometrically with design complexity so - KISS. We had a machine that had to overcome gravity to operate and I always figured that, should gravity fail, trial lawyers and product liability suits would be the least of our concerns.

If a cable snaps, it's likely to decapitate an occupant (or occupants) of the tram so a broken cable is unacceptable and having redundant cables is, IMO, a serious design flaw. I think a single, adequately over-sized cable, driven by a large diameter drum and inspected regularly (on a rigid schedule) is better.

The drum then becomes the s>I don't think that all the suggestions are necessarily goldbergian.

Rohan, Thanks for the comments. Yes, you have pretty much summed up the situation. What you speak of would be a good solution if there was an easy way to determine the overspeed condition without any expensive electronics or hydraulics.

The dampening of the cable is something I hadn't considered, but is a potential risk. I'm hoping that since there aren't extreme loads on the cables (500 lbs per cable at 30 degrees) that the whiplash effect would be minimized, but I could be wrong.

Thanks! Jeff

rhamer wrote:

Thanks Robert. Yes, I'm thinking more and more of a solution like this, but I'm not as concerned of a breaking cable as I am the freespool event. Each cable is rated at 8,000 lbs. The cart fully loaded would be around 1,000 lbs at 29 degree slope. At this slope, the load would be about 500 lbs per cable which is 1/16 the max load of a single cable. So if one cable were to fray and break, the other can handle the load easily. I have them going through pulleys, but there is also a backup attachment to the cart, so if one cable breaks it will still have an attachment to the other cable.

But, it is possible I think to have the 3rd cable as you mention with a springed arm of some sort that would "grab" the cable if the tension of the primary cables released. I suppose I could adjust this "trigger point" by adding additional springs such that the trigger tension was almost to the point of catching. If I were to set it up such that the trigger locked and wouldn't let go, I'd run the risk of someone jostling the cart when they get on, releasing some tension and triggering the lock. Perhaps a scaled approach that the less tension the greater the "grab" would work, if that makes sense.

Robert L Bass wrote:

pulley on the end to take up slack in the cable on the

strut to draw it up toward the tram. Slack feeding out

it will always remain if there is tension on the

either be on a third cable or on the wheels. A cable

no electronics and should be easy to maintain. Trial

arm and the brake.

stage, I would select equipment rated to hold 10 times

stop a load even as it gained momentum before

momentum as quickly as on a deadfall. However, depending on

the rails, the tram could accelerate, perhaps to the

the tram, number of people who will be riding at one

sounds as though you've given this careful

Thanks Dave. Each cable is rated at 8,000 lbs and the cart is 1,000 lbs fully loaded on a 29 degree slope, so yes, 2 cables is way overkill, but since our wheels were on round pipe we thought it would also be beneficial to have the alternate cable to prevent listing from one side or the other as the cable changed positions on the drum.

We originally were going to use the other drum as a counterweight, but we discovered our motor and gearbox powered everything so easily with no strain (even the lights in the house don't flicker when it starts, like my HVAC compressor does), so we went with the 2 cable design. We weren't as concerned of overloading a single cable as much as we were long therm exposure to the elements and constant use. I guess a wear break was a larger concern. So, IMO, I disagree with it being a design flaw, although it may be over designed.

I do agree with the KISS methodology however and having it constantly inspected.

We did have the drum assembly professi> I didn't say they were _all_ Goldbergian.

And consider that most funiculars have dual trams connected to each other by a single cable that passes through a pulley (capstan) at the top end of the ramp. As one tram climbs, the other descends. The weight of the riders is probably minimal compared to the weight of the trams so the design is inherently safe with the weight of the ascending tram limiting the velocity of the descending tram.

What speed do you think the fully loaded cart would reach if it rolled from the top of the ramp unrestrained?

Jon, I'm not sure. It currently travels at 1.3 ft/sec up and down and is set at half speed. When I get more time (its not my fulltime residence have to travel 5 hours to work on it) I want to crank up the speed to near full speed which should be a little over 2 ft/sec. The trick is that I have to adjust the start and stop microswitches to keep it from crashing to the bottom or the top with the increased speed.

So, I would assume 3 ft/sec would be c> What speed do you think the fully loaded cart would reach if it rolled from

Well, given that you now want something to paste on to your design, add a heavy flywheel to the drum.

Problem with that is that it would increase the coast time of the cart going down. Currently its about 3 feet when the "normal stop" command is sent.

But something on a similar vain that could work is something with a constant drag like the air compressor idea that was suggested, or even a squirrel cage fan underneath the drums if geared with a bicycle chain. The motor could overpower the additional going down

Actually, I was thinking that you could use one or both of the main cables to operate the brake. The third cable idea was from another gentleman. Given what you've said about the weight, slope and cable strength, I seriously doubt you'll have a broken cable issue. If you use the operating cables to keep the brake pulley / lever taught, the weight of the tram will keep anyone from jostling the cable off the pulley. You can use a catcher (U-shaped cover) above the pulley so that the cable is trapped at the top if this is still a concern.

The main reason I like this idea (really from Doug's suggestion) is it's simple. It require no electronics and no electrical power to operate. Also, you could easily build in an over-ride to allow the tram to descend in the event of an erroneous safety system deployment or a single cable failure. A manual lever could be operated to allow you to walk the tram down the hill.

operate the brake. The third cable idea was from

strength, I seriously doubt you'll have a broken cable

taught, the weight of the tram will keep anyone from

above the pulley so that the cable is trapped at the top

simple. It require no electronics and no electrical power

descend in the event of an erroneous safety system

allow you to walk the tram down the hill.

Robert, what you said about Doug's idea makes complete sense. I was initially thinking of something like this pre-construction, but the cable rigidity discouraged the concept. But That was when I was thinking of assuming there would be total loss of tension and gravity would "flip the switch" so to speak.

To describe the way the cable is mounted, imagine the floor lifted off as you are on the cart facing down hill. You would see a cable under your left foot going to the down hill corner, to a pulley, then over to the side pulley on the right and then back under your right foot. So in a sense, the cable makes a large "U" (upside down that is). If I were to add a center pulley such that it made a "W" attached to a spring, I could as you and Doug say, use springs and the cart's weight to adjust to the condition of freespool or just normal use such that a small loss in tension could grab onto the cable.

Another benefit to it is that I could use this method to hold the cart in place as I work on it, although I would probably count on a little more than just the grapple.

The trick will be to come up with a method to grab the wire. I've thought of an eyebolt with a spring (like a valve spring) to grab it in sheer, or perhaps a rod with two pins that torques and binds the cable. I think the best would be a go kart brake. They are mechanical (non hydraulic) and have a lever built in. I would probably need to wear a groove in the pads to increase the surface area.

Sounds like a plan.

No, that would be the speed at which I would assume its a runaway. If you mean by terminal velocity the maximum speed it would reach on a runaway condition, I don't have a clue.

J> 3 ft/sec is the terminal velocity?