18 replies to this topic

[Kelly]

Sheave loads: Weight on a sheave is controlled by design/code loads. Typically the average is not more than 500 lbs. per sheave.
As loads approach 250 to 0 lbs. a compression assembly is used. If the load exceeds 500 lbs. more sheaves per assembly and or larger diameter sheaves are used.
If negative loads are required, hold down assemblies are used.
Some hold down assemblies are support assemblies turned upside down.
Minimum loads on sheaves are needed to counteract side loads such as swinging carriers and wind.
Maximum loads are given so that mechanics can perform maintenance safely on cross arm components.
Typical loads:
2 sheave assembly = 1000 lbs. total
8 sheave assembly = 4000 lbs. total

Ryan B

[KZ]

is the hold down both compression and regular sheaves?

[Allan]

Hold down sheaves are compression sheaves - is that what you mean? Some compression sheaves have a smaller side plate so the grips don't hit it and cause a really bumpy ride.

[liftmech]

Holddown (or depression) sheaves area those that the rope passes under, while support sheaves hold the rope up. A compression assembly has both support and depression sheaves, and is used in a neutral load situation (see Ryan B's post above). A neutral load is one where the rope has a tendency to float off the sheaves when a chair is loaded (in a depression situation) or when the line is empty (in a support situation). A good example of a heavy load requiring more sheaves is a breakover or 'fan' tower setup, where there are many towers, all with the maximum amount of sheaves. I think Allan posted a good example pic a while back.

[Kelly]

Definitions:

Compression Assemblies: An assembly of sheaves in certian combinations that support and hold down the haul rope. This type of assembly controls the haul rope for both positive and negative loads. Also called trap assembly.

Support Assemblies: An assembly of sheaves that hold up the haul rope from the bottom side. Also called breakover assembly.

Hold down assembly: An assembly of sheaves that hold down the haul rope from the top side. Also called depression assembly.

A compression assembly is common in steeper spans with little slope change, flat spans with large slope change, and at loading towers. However it is common to use support/holdown terminology with large assemblies with only 1 opposing sheave.

I believe compress means to squeeze on all sides or all "available" sides.

I have submitted these to NWS for the glossary.

liftmech - seems like these definitions might work for most people, any comments?

Ryan B

[Allan]

Is this the pic? :)

Attached File(s)

•  MotherLode___Breakover_towers.JPG (192.65K)
  Number of downloads: 27

[Allan]

OR was it this one

Attached File(s)

•  DCP_1210.JPG (404.2K)
  Number of downloads: 23

[Allan]

This is a picture (poor quality) of our hold-down towers... The farthest portal tower has 20 sheaves, while the closest has 10.. I'll try and get some closer pics...

Attached File(s)

•  MotherLode___Portal_Towers.JPG (191.47K)
  Number of downloads: 38

[KZ]

oh, i always thought compression was just pushing down. Thanks ryan.

[Dr Frankenstein]

If I translate word-to-word from the French, compression assemblies means depression assembly.

Weird.

[coskibum]

here is a hold down assembly:



here is a compression assembly:




here is a support assembly:



i learn more visually so hopefully this will help!

This post has been edited by coskibum: 02 December 2003 - 05:53 PM

[liftmech]

Yeah, Ryan, those are the same definitions I've heard from most people. It's another area where two or more terms for the same thing leads to confusion... :help:

[Allan]

I learned the sheave assemblies with both support/depression sheaves as a combination sheave assembly!

[Dr Frankenstein]

Me too.

[KZ]

i thought depression was what the real hold down is

[Allan]

This is what I learned...

Support - any tower where the grips pass over the sheaves
Depression - any tower where the grips pass under the sheaves
Combination - any tower with both support/depression sheaves.

But RyanB's definitions make more sense!

[Kelly]

Combination does have a certain ring to it. However here are a few examples where the word combination can be confusing if you're not actually looking at the assembly.

-Combination of assemblies: may be of the same style such as 4 support assembly + 2 support assembly.
-Combination of combination assemblies.
-Grouping of combination assemblies which could be a combination group.

It seems like once the definition of compression is defined: to squeeze on both sides, combination works best as term to describe a nonspecific grouping of assemblies and or sheaves.

Boy I'm getting depressed about the compression of support combination assemblies.

Ryan B

[liftmech]

:---:

[KZ]

Nice