Well, actually, K3NA was just reporting from the "Handbook of Rigging"
regarding sheave diameters. To further illuminate the subject, I will
just quote from the chapter 10, "Blocks, Reeving, Sheaves, and Drums":
"If a wire rope is used with inadequately sized sheaves, the severe
bending stresses imposed on the rope will cause wires to break from
fatigue, even though actual wear may be slight. One of the fastest ways
to destroy a [wire] rope is to run it over small sheaves. The excessive
and repeated bending and straightening of the wires leads to premature
failure from fatigue.
"Small sheaves will also accelerate wear of both rope and sheave
groove. Since the pressure per unit area of rope on a sheave groove for
a given load is inversely proportional to the size of the sheave, the
smaller the sheave diameter, the greater the rope pressure per unit area
on the groove.
"To determine the unit radial pressure between a rope and a sheave,
use the formula P = (2 L) / (D d), where:
P = unit radial pressure, lb/in^2
L = load on rope, lb
D = tread diameter of sheave, inches
d = nominal diameter of rope, inches.
The allowable unit radial bearing pressures of various ropes on
different sheave materials are given in Table 10.2."
(Taking the example 100 lb load I gave in earlier messages, and using
1/4" diameter wire rope, we can calculate pressure for a 8.5" tread
diameter and a 2" tread diameter. For the big sheave, P = 94 lb/in^2.
For the small sheave, P = 400 lb/in^2. Remember, the 100 lb load of
this example is a STATIC load from the dead weight of the antenna, with
no acceleration force to start the load moving up the tram or additional
load from a breeze or tag lines or bounce or other factors. Table 10.2
provides these values for allowable unit radio bearing pressure on
various sheave materials for regular lay 6x19 wire rope -- the kind that
wood: 250 lb/in^2
cast iron: 480 lb/in^2
carbon steel: 900 lb/in^2
chilled cast iron: 1100 lb/in^2
manganese steel: 2400 lb/in^2
Continuing with the text...)
"If the unit radial pressure exceeds these maximum values, the
material from which the groove is manufactured is too soft for the
operating conditions. Therefore rapid wear of the grooves will result.
"The sheave diameter can also influence the rope strength. When a
wire rope is bent around a sheave, there is a loss of effective strength
due to the inability of the individual strands and wires to adjust
themselves entirely to their changed position. The rope strength
efficiency decreases to a marked degree as the sheave diameter is
reduced with respect to the diameter of the rope.
"Wire rope manufacturers have established standards for sheave sizes
that should be used with various rope constructions (see Table 10.3).
Always use the maximum possible sheave diameter that the lifting
equipment will carry."
(Table 10.3 gives recommended and minimum D/d ratio for various kinds of
wire rope construction. 6x19 Seale wire rope has a suggested D/d ratio
of 51 and a minimum of 34. For d=0.25", the minimum D is 8.5 inch. In
this regard the Handbook is quite conservative. See also
"Hard sheave surfaces offer the best bearing surface for wire rope,
thus prolonging sheave and rope life. If a sheave is forged or cast
from a material softer than the wire rope, the sheave life and the wire
rope life will be shortened. The sheave will have a tendency to take on
the impression of the rope, causing scoring and corrugation of the line
"Cast manganese steel sheaves offer the ultimate in sheave material.
This surface actually hardens to the use of wire rope and provides
greatly extended sheave life and increased service time of the wire ropes."
Suppliers: Wire rope snatch blocks are easily located via Google.
Snatch blocks usually are more expensive and heavier that blocks that do
not open, prompting one to think: "How difficult is it, really, to tread
the tram line through the block before anchoring to the tower or ground?"
Sheave data & ordering can be found at, for instance:
and suppliers can be located via Thomas' catalog (www.thomasnet.com)
This equipment is not cheap. But, you buy it only once! And it can
be a great investment for a club.
For fiberous rope and blocks that work with fiberous rope, I (and other
contesters) have ordered on-line from CMC Rescue Equipment. Their
prices are reasonable and the equipment is outstanding. Again, this is
a once-in-a-lifetime purchase of blocks and carabiners. I found myself
discarding a lot of inexpensive hardware store junk after my first
experience working with CMC materials on a tower -- I had not realized
how difficult life was with the stuff I was using before. They also
have climbing gear, including fall protection and work positioning
harnesses for tower work.
-- Eric K3NA
on 06 Dec 21 Thu 21:55 Dick Green WC1M said the following:
> Can someone direct me to a good supplier of metal sheave snatch blocks
> suitable for tramming?
> K3NA says I need at least an 8.5" wheel, but I'd like to revisit the
> calculations on that. I think they're based on the rope making at least a
> 90-degree turn around the sheave, which doesn't happen in the case of the
> tram. The wire rope does bend around a small portion of the wheel's radius,
> but not much. I suspect a wheel in the 2"-4" range would be more than
> adequate. Comments? What are people out there using for pulleys?
> 73, Dick WC1M
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