CABLE TRAY PULLS TAKE PLANNING
Designing a cable tray installation? Think about your sheaves and rollers.
- Published in Southwire's Power Cable Update newsletter in June 1997
- Reprint permission granted
Pulling wire into trays? Even though the cables ride on sheaves and rollers, your project shares some concerns with a conduit pull-the need for careful planning is the biggest one.
"Damage caused by a poorly planned pull — can shorten cable life," says Sid Ticker, senior applications engineer. "Corner sheaves and support rollers are two important factors in a good cable tray installation."
Be Careful With Your Bends
A change in direction — horizontal or vertical — can be a trouble spot in any pull. Think about sidewall pressure. Sheaves will get cable around a corner with safe sidewall pressures, if they're used correctly.
Because individual sheaves have small diameters, use at least one sheave for every 20 degrees of bend.
In particular, never try to navigate a 90-degree bend with just three sheaves. Make sure all sheaves are in good condition and well lubricated. If the sheaves don't roll freely, every bend will multiply pulling tension-and sidewall pressure. Don't assume that just because a sheave has very little friction it doesn't add to tension. Even if the sheaves roll freely, it takes some force to bend a heavy cable around each one. As a rule of thumb, add 100 to 150 pounds per bend to the pulling tension for heavy cables.
Align your sheaves before the pull. Put a little tension on the pulling rope and use that as a guide. Check the sheaves carefully during the pull. Even with pre-alignment, they may need adjustments.
How Many Support Rollers Do You Need?
Rollers support the cable so it won't drag on the tray as it moves. Fifteen feet between rollers is usually tolerable, but 10 feet may be better. Putting rollers closer together decreases sag between rollers, and that decreases pulling tension.
You can approximate roller spacing at a specific tension with this formula:
S = 2.83 H T/W
Where:S = distance between rollers in feet (meters)
H = height of top of rollers above the tray, in feet (meters)
T = tension in pounds (kg.), and
W= weight of cable in lbs./ft. (kg./m.)
Those rollers need support. The cable tray isn't designed to do that. Allow for both vertical and horizontal loads, and remember to leave room for support structures in your tray layout. In bends, you can put a sheet of plywood on top of the tray to support the sheaves.
If you're pulling armored cable, pin the armor to the conductors to keep the conductors from moving out of the armor. Use bolts or nails that go through the armor and conductor core to lock them together. You can also use a compression grip on the armor in conjunction with the conductor pulling eye or bolt.
"Every pull is different," says Ticker. "Sheaves and rollers are just two of many factors. Attention to all the details pays off." |
