Ball Screws 101
BALL SCREWS 101
Backdriving:
Normally, ball screws are used to convert rotary motion into linear motion. Backdriving is the result of the load pushing axially on the screw or nut to create rotary motion. All ball screws, due to their high efficiency, will backdrive. The resulting torque is known as “backdriving torque” and is the torque required to hold a load in position.
CAUTION - When using ball screws, applications should be analyzed to determine the necessity of a brake, especially when the possibility of injury may occur.
Backdriving Torque:
Due to the efficiency of a ball screw, a load applied to the ball nut will generate backdriving torque on the ball screw. The torque required to hold the load in position can be calculated by the following formula.
Back Driving Torque Formula:
Td=(P*L*e)/(2*PI)=.143 P*L
WHERE:
Td = Drive Torque (pound-inches)
P = Load (lbs.)
L = Screw Lead (inches/turn)
e = Ball Bearing Screw Efficiency (90%)
PI=3.1415926535897932
Backlash (lash) is the relative axial movement between a screw and nut without rotation of the screw or nut. Lash in ball screws will remain constant during normal use.
Ball Circle Diameter:
The diameter of the circle generated by the center of the bearing balls when in contact with the screw and nut.
Bearing Ball Circuit:
The closed path that the bearing balls follow through the ball nut. Ball nuts may have one or more circuits.
Compression Load:
A load that tends to “squeeze” the screw.
Driving Torque:
Driving torque is the amount of torque required by the ball screw to move a load. To simplify this calculation a “torque to raise one pound or one kN” value is provided in the technical data for each ball screw size.
To determine the required torque to move a load, multiply the load to be moved by the “torque to raise one pound or kN”. For more information on drive torque, see the application selection example.
Dynamic Load:
The thrust load in pounds which, when applied to the ball nut and rotating screw assembly will result in a minimum life of 1,000,000 inches of travel. Metric screw designs are per ISO 3408 and show the load ratings in kilonewtons for 1 million revolutions of the screw.
Efficiency:
The low coefficient of friction of the rolling elements of Ball Screws and Nuts results in an operating efficiency greater than 90%.
Land Diameter:
The outside diameter of the screw. This diameter is less than the ball circle diameter.
Lead:
The axial distance the nut advances in one revolution of the screw. The lead is equal to the pitch times the number of starts. (Lead = Pitch X Starts)
Lead Accuracy:
Lead accuracy is the difference between the actual distance traveled versus the theoretical distance traveled based on lead. For example: A screw with a .5 inch lead and ±0.001 inch per foot lead accuracy rotated 24 times theoretically moves the nut 12 inches.
24 Revolutions X .500 inches per revolution = 12.000 inches of travel with a Lead accuracy of .001 per foot, actual travel could be from 11.999 to 12.001 inches.
Life:
A ball screw assembly uses rolling elements to carry a load similar to an anti-friction (ball) bearing. These elements do not wear during normal use. Therefore, ball screw life is predictable and is determined by calculating the fatigue failure of the components. Proper lubrication, regular maintenance, and operation within specified limits will allow Ball Screws to operate to the predicted life.
Load Carrying Balls:
The bearing balls in contact with ball nut and ball screw sharing the load.
Overturning Load:
A load that tends to rotate the nut around the longitudinal axis of the screw.
Pitch:
The axial distance between threads. Pitch is equal to the lead in a single start screw.
Preload:
Preload is an internal force introduced between a ball nut and screw assembly that eliminates free axial and radial lash. Preloaded assemblies provide excellent repeatability and increased system stiffness. Preloading is achieved either by using two nuts and forcing them apart or by shifting the circuits within a single nut. Nook Industries has a variety of preload ball nut designs available.
Return Guide:
When bearing balls circulate in a ball nut, a ball enters the ball path between the nut and screw carrying the load one or more turns around the screw. The bearing ball is then picked up and returned to the beginning of the circuit through the return guide.
Root Diameter:
The diameter of the screw measured at the bottom of the thread. This is the diameter used for column strength, critical speed calculations and end machining considerations.
Screw Starts:
The number of independent threads on the screw shaft; typically one, two or four.
Selective Fit:
The axial movement between a new SBN or SGN ball nut and screw will range from 0.003" to 0.015" depending on size. When less than standard lash is desired, SBN and SGN ball nuts can be custom-fit to a specific screw with selected bearing balls to minimize lash to 0.003" to 0.005" depending on ball size. Select fitting may result in lower life.
Side Load:
A load that is applied radially to the nut.
Static Load:
The maximum thrust load – including shock – that can be applied to the ball nut without damaging the assembly.
Straightness:
Although Ball Screws are manufactured from straight, cylindrical material, internal stresses may cause the material to bend or yield. When ordering random lengths or cut material without end machining, straightening is recommended. Handling or machining of screws can also cause the material to bend or yield. Before, during and after machining, additional straightening is required.
Tension Load:
A load that tends to “stretch” the screw.
Thrust Load:
A load parallel to and concentric with the axis of the screw.
www.widgitmaster.com
It's not what you take away, it's what you are left with that counts!