Ballscrew Selection Guide

Printer Friendly PDF Version

Specification
Equipment: Transfer Table
Screw Orientation: Horizontal
Load Supported on Dove Tail Ways: .20 Coefficient of friction
Load is 2500 lbs. Max (combined weight of product and table)
Stroke Length: 38"
Travel Rate: 600 inches per minute (Max.)
Input RPM: 2400
Duty Cycle: 20 cycles per hour, 16 hours per day, 250 days per year
Required Life: 5 years

Step 1: Determine Required Life (Inches)
38"/stroke * 2 strokes/cycle * 20 cycles/hr * 16 hrs/day * 250 days/year * 5 years = 30,400,000 inches

Step 2: Determine Thrust Load on Ball Screw
Multiply the thrust load by the coefficient of sliding friction (for horizontal application):
2500 lbs. * .20 Coefficient of Friction = 500 lbs.
Use this load for life calculations.
(If load varies during the stroke or cycle, an equivalent load calculation can be utilized page 7 of catalog)

Step 3: Determine Required Ball Screw Dynamic Axial Loading to Achieve Required Life (page 7 of catalog):
Using formulas on page 7, input the 500 lbs. thrust load (Or equivalent load) and the required life. The result is the minimum rated load for a ball screw to achieve the required life. Rated Load Actual life of assembly under actual load

Step 4: Determine Lead of the Screw:
Travel Rate (pg 10 of catalog):

USE THIS QUICK REFERENCE CHART TO SELECT APPROPRIATE BALL SCREW

Model	Screw Diameter x Lead	Screw Rated Load	Screw Minor Diameter	Catalog Page Number	Model	Screw Diameter x Lead	Screw Rated Load	Screw Minor Diameter	Catalog Page Number
R10	.375 x .125	150	0.300	22	R45, R47	1.150 x .200	2,450	1.020	38
R11	.375 x .125	300	0.300	23	R46*	1.150 x .200	490	1.020	38
R15*	.375 x .125	25	0.300	22	R48	1.063 x .625	3,300	0.925	39
R16*	.375 x .125	50	0.300	23	R50	1.500 x .500	9,050	1.260	40
R20, R23	.500 x .500	850	0.400	24-25	R50A, R51A	1.500 x .500	12,900	1.260	41
R21, R22	.500 x .500	140	0.400	24-25	R53, R54	1.500 x .250	4,250	1.375	42
R30, R31	.631 x .200	825	0.500	26	R54A	1.500 x .250	6,400	1.375	43
R30A	.631 x .200	1,650	0.500	27	R55, R56	1.500 x 1.00	8,000	1.140	44
R32*	.631 x .200	170	0.500	26	R57	1.500 x .4375	10,500	1.140	45
R34	.750 x .200	1,900	0.650	28	R58	1.500 x 1.875	7,350	1.190	46
R35	.750 x .200	950	0.650	29	R60, R63	2.250 x .500	19,800	1.860	49
R36	.750 x .200	160	0.650	29	R60A	2.250 x .500	29,700	1.860	50
R37	.750 x .500	3,400	0.630	30	R61	2.000 x 1.00	22,500	1.730	47
R38*	.750 x .500	600	0.630	30	R62	2.000 x .500	18,000	1.730	48
R40, R41	1.000 x .250	1,625	0.840	31	R70	2.500 x .500	22,000	2.220	51
R40A	1.000 x .250	3,250	0.840	32	R71	2.500 x 1.00	26,500	2.220	52
R40RF, R41LF	1.000 x .250	3,250	0.840	34	R74	2.500 x .250	6,300	2.320	53
R40B	1.000 x .250	4,500	0.840	33	R75	2.500 x 1.50	32,500	2.100	54
R42	1.000 x .250	3,450	0.870	35	R80	3.000 x .660	42,000	2.480	55
R43	1.000 x .500	4,250	0.870	36	R90, R91	4.000 x 1.00	85,000	3.338	56
R44	1.000 x 1.00	2,300	0.870	37	* DENOTES STAINLESS STEEL MODELS

Ball Screw Selection:
Load rating: Requires ball screw operating load capacity of 1,561 lbs. Minimum smallest diameter screw with 1,561 lbs. (min.) operating load and a .250" lead is the R40 (page 31 of catalog)

Step 5: Calculate Length Between Bearing Supports:
Length between bearings = Stroke length + Ballnut length + Desired over-travel

38" stroke + 2.347 nut length (page 31 of catalog) + 1" over-travel = 41.347" between bearings
(use this length for column load and critical speed calculations)

Step 6: Calculate End Fixity Based on Critical Speed Limits (page 7-8 of catalog):
Using formula for Critical Speed, rearrange to solve for Fe (End Fixity Variable)

Equations below will solve for the minimum end fixity factor based on Travel Rate (600 in/min.)

Thus a Fixed-Simple (Fe = 1.47) is the proper selection

Step 7: Actual Calculated Critical Speed:
This calculated critical speed is based on the Fixed-Simple end fixity arrangement. It is the maximum safe linear speed with this mounting arrangement, screw model and between bearing supports distance. If greater speed is required, a Fixed-Fixed arrangement can be used, recalculate maximum speed based on a fixed-fixed end fixity configuration (Fe = 2.23).

Step 8: Calculate Critical Ball Speed (DN) (page 8 of catalog):
Critical ball speed is the maximum safe linear speed of this model regardless of screw length. In this example DN should not be less than 687" per minute.

Step 9: Calculate Column Load Limit (page 8 of catalog):
This calculated column load is the maximum safe compression load allowable based on mounting arrangement, screw model and distance between bearings. In this example the calculated column loading should be greater than 500 lbs. (Step #2).
column load limit

Step 10: Calculate Drive Torque (page 9 of catalog):

Step 11: Calculate H.P. Required at Constant Velocity (page 10 of catalog):

Step 12: Specifying Proper Ball Screw Assembly (page 31 of catalog):

Step 13: Refer to the 2D and 3D downloadable drawings from rockfordballscrew.com

Printer Friendly PDF Version