The Ratio of Cable or Wire Rope Diameter to Pulley or Sheave Diameter

 This is often referred to as the D:d ratio (Diameter of the pulley or sheave to the diameter of the cable expressed as a ratio) e.g. a 500mm diameter sheave compared to a 5mm diameter cable would be a 100:1 D:d ratio.

The larger the pulley or sheave diameter compared with the diameter of the cable the better the service life of the cable will be.

The sheave profile is also very significant in improving cable life with a "U" shaped profile supporting the cable being the optimum shape.

The material of the sheave or pulley can also significantly improve cable life.  

The D:d ratio is the size ratio between the diameter of the sheave (D) and the diameter of the wire rope (d). The larger the D:d ratio, the better the fatigue performance of the wire rope will be.

D:d Ratio Drawing

Due to the difference and varying complexity of wire rope constructions it is impossible to accurately predict the effect of variations in the ratio but we can offer some guidelines.

Important assumptions:

1. The wire rope is a suitable diameter and construction for working over sheaves.
2. The sheaves are made from a suitable material with the correct type of profile.
3. The sheaves are well maintained and free running.
4. The wire rope has been spooled onto the drum correctly and that there is no twist in it's length.
5. The wire rope is working within it's stated working load limit (WLL).

Important points of note:

1. Correct lubrication will significantly increase the wire rope's life.
2. Environmental variations will have a positive/negative effect on the wire ropes life.

The table below shows the recommended ratios (desirable minimum and critical minimum) based upon cable diameter and construction.

RATIO OF CABLE DIAMETER TO PULLEY DIAMETER (D:d Ratio)
  Desirable Minimum (mm) Critical Minimum (mm)
  42:124:112:1 28:118:110:1
Cable Diameter
in Inches
Cable Diameter in mm6x7
or
7x7
6x19
or
7x19
6x31
7x31
6x37
 6x7
or
7x7
6x19
or
7x19
6x31
7x31
6x37
1/320.834-------- 22--------
 142-------- 28--------
3/641.25029---- 3422----
 1.56336---- 4227----
1/161.66738---- 4529----
 28448---- 5636----
3/322.3810057---- 6743----
 312672---- 8454----
1/83.213477---- 9058----
5/32416896---- 11272----
3/164.8202115---- 13486----
 5210120---- 14090----
7/325.56234133---- 156100----
 6252144---- 168108----
1/46.3526715276 17811464
 7294168---- 196126----
5/167.933219095 22114279
 833619296 22414480
 9378216108 25216290
3/89.5399228114 26617195
 10420240120 280180100
7/1611462264132 308198110
 12504288144 336216120
1/212.7533305152 356229127
 13546312156 364234130
9/1614588336168 392252140
5/8"16672384192 448288160
 18756432216 504324180
3/419798456228 532342190
 20840480240 560360200
7/822924528264 616396220