28 replies to this topic

[liftmech]

Kicking Horse, on Feb 21 2005, 07:01 PM, said:

Another Quote from the tramway rules.
I thought all lifts are suppose to have 3 different brakes?

In fact the it says  the aerial lift shall have the following friction type brakes

Service Brake
Drive Sheeave Brake
Rollback Device

I see that as 3 different brake systems? 
I'm puzzled
<{POST_SNAPBACK}>

ANSI and CSA require the same braking systems on lifts; they read almost identical each other.
What that rule states is simply that when you hit a stop, the service brake shall set. If it doesn't, the system shall automatically initiate an emergency stop. That doesn't mean you only need those two systems, merely that failure of the one shall cause activation of the other. An example: I can disable the service brakes on the Flyer with a special tool (and once a week I test this function). When I hit a stop, the brakes will remain open, the system will see that (there is a switch on each brake that should open when the brakes close) and dump the drive sheave brake. (The rollback will dump 10 seconds later, but that is unrelated.)
In practical terms, most aerial lifts have four brakes. The service brake is the primary holding device, hence the term service and either closes once the lift stops or helps it to stop. The high-speed backstop is located in the gearbox, and is a one-way device which only lets the drive shaft rotate in the forward direction. Some lift manufacturers factored this inot their designs and had the load of the lift rest briefly on the backstop upon start, but that is no longer allowed by code. The third brake is the drive sheave brake, which is what applies on an emergency stop (it is commonly called the emergency brake for this reason). The fourth and last is the rollback brake or rollback device. This can take the same form as the drive sheave brake, or perhaps it is a piece of steel that drops into the path of the bullwheel spokes. This is the last-resort brake to keep the lift from going backwards. When the gearbox/bullwheel interface sheared onS-lift, the reverse rotation of the bullwheel pulled the drop dog down and it jammed on the spoke, stopping the lift.

[Boeinglover]

I'm not getting that. Explain in better detail if you can.

This post has been edited by Boeinglover: 22 February 2005 - 07:54 AM

[Outback]

Liftmech,

Do you need a job?

[Allan]

Here's the table for that... Z98 :)
T=Time, V=Velocity, A=Rate of decel, m/s^2

Stopping Time (seconds): t=v/a
Stopping Distance (meters): s=v^2/2a

Maximum allowable decel rate = 1.5m/s squared (shortest time or distance)
Minimum alllowable decel rate = .45m/s squared(longest time or distance)

2.5m/s (500 fpm) minimum stop time...
2.5/1.5=1.67s
maximum...
2.5/.45=5.56s

Don't ask me about the math, the CSA did it.

[Allan]

Here's the little cheat sheet.

Attached File(s)

•  Stop_Distancesm.jpg (181.03K)
  Number of downloads: 36

[Duck]

Boeinglover said:

I'm not getting that. Explain in better detail if you can.

Try this.

-Iain

This post has been edited by Duck: 22 February 2005 - 09:31 AM

[liftmech]

Outback, on Feb 22 2005, 09:15 AM, said:

Liftmech,

Do you need a job?
<{POST_SNAPBACK}>

Why, you have one? :D

[Matt]

Here's a useful formula for finding the stopping distance. It's derived from this formula:

2AX = Vf^2 - Vi^2
Where:
A is acceleration
X is distance
Vf is final velocity
Vi is initial velocity

Because we're looking for a final velocity of 0:

2AX = -Vi^2

Solving for X:

X= (-Vi^2)/2A

Another thing to note it that this formula, as well as the others mentioned are for constant acceleration only.

This post has been edited by Matt: 01 April 2005 - 10:19 AM

[Aussierob]

Doppelmayr have gone away from backstops/rollback dogs in their gearboxes. They use a brake on the bullwheel instead. This is much simpler as they can run in reverse. Pomas don't go backwards so a rollback dog in the box is easier.

Nice work on explaining stop time/distance guys. At a load test it's always the thing that takes most time to get right.