Carl Stahl Sava Industries

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Cable Design Guide

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Design Info
Guidelines for Cable Assembly Selection

1. Select a cable that has a minimum breaking strength of 10 times the working load.
2. Should a pulley be utilized, see chart on Pulley Guidelines Page, for proper pulley diameter.

AFTER you have determined the maximum tension on the cable and selected the cable diameter and pulley for your system, apply the numbers to the formula below to determine the cable load factor. Read anticipated cycle life from chart.

NOTE: A system frequently cannot be designed for optimum conditions. The formula and chart show that a trade-off can be made between cable size, pulley size and life.

Cable Fatique Life Chart

NOTE: The numbers on this chart were determined under laboratory conditions and are intended to serve as a guide. Since there is considerable variation in application, we recommend actual life tests for your intended use.

Two kinds of stretch occur in cable – constructional stretch and elastic stretch. They are due to two different causes.

Constructional Stretch
When strand and cable are made, the load at the closing head is light. Therefore, there are small clearances between the wires and strands, and between the strand and the core. The application of initial load causes wires and strands to seat properly, and a slight overall elongation of the strand or cable accompanies this section. The amount of constructional stretch is not constant for all cables – it depends on such variables as type of construction, length of lay, and other factors, including the load applied.

Elastic Stretch
Elastic stretch is the actual elongation of the wires of a strand or a cable. This is caused by the application of a load, up to the yield point of the metal, and the stretch is approximately proportional to the load applied. When the load is released, strand or cable subjected to elastic stretch returns to its approximate original length, providing the stretch has not reached the yield point of the metal.

Removal of Stretch
Where the elimination of as much stretch as possible is important, the cables or assemblies can be proof loaded to remove most of the constructional stretch. For assemblies, this process also verifies the holding power of the terminals. Proof loading is usually done by applying a 60% load to the cable or assemblies. This load is based on the minimum breaking strength of the cable or fittings, whichever is lower. Handling the cable as little as possible after prestretching helps eliminate putting constructional stretch back in.

FORMULA (see example)

Cable Factor = (Tension on Cable / (Bare Cable dia. x Pulley Root dia.))
7x7 Construction Nylon Coated
7x7 Construction
Nylon Coated
Curve for part nos. 2014SN, 2018SN, 2027SN, 2031SN, 2036SN, 2048SN, 3048GN, 2064SN, 2094SN, 3064GN, 3094GN
3X7 Construction Nylon Coated
3X7 Construction
Nylon Coated
Curve for part nos. 2009SN, 2012SN, 2030SN
7x19 or 7x49 Construction Nylon Coated
7x19 or 7x49 Construction
Nylon Coated
Curve for part nos. 2019SN, 2024SN, 2032SN, 2037SN, 2038SN, 2045SN, 2047SN, 2050SN, 2054SN, 2065SN, 2095SN, 3065GN, 3095GN
7x7 Construction Uncoated
7x7 Construction
Curve for part nos. 2018, 2027, 2031, 2036, 2046, 2048, 2064, 2094, 3048, 3064, 3094

Determine the cable size, pulley diameter, and cycle life for a counterweight system working with a load of 15 lbs. Anticipated life of cable should be 2,000,000 cycles.

Step 1 Select a cable from the Stainless Steel Cable, Nylon, and Vinyl Coated page, which has a breaking strength of 10 times the load. Cables 2036, 2037, and 2038, .036˝ diameter, qualify.
Step 2 Multiply cable diameter .036˝ by 40 for pulley groove diameter. .036˝ x 40 = 1.44˝
Step 3 Cable Factor = 15 / ( .036 x 1.44 ) = 289

Applying this factor to the Cable Fatigue Life Chart, it is seen that a 7x7 bare cable will have an expected cycle life of only 400,000 cycles, which is not sufficient. When a nylon-coated cable (2036SN) is selected, the Cable Factor line will not intersect the curve on the cable for a 7x7 or 7x19 coated. The 7x7 coated cable (2036SN) is less expensive and would be the logical choice.

Elastic Stretch Characteristics of Sava Cable