Freewheeling Ball Screws (sometimes referred to as epicyclic or planetary Ball Screws) are used with stop pins at each end of the travel. When the stop pin engages the ball retainer, the linear motion stops. However, the screw can continue to rotate (freewheel) with the stop pin engaged. When the screw rotation reverses, linear motion occurs away from the stop pin until the stop pin at the opposite end of the screw is engaged.
Freewheeling Ball Screws offer the major advantage of replacing limit switches or other “soft stops” with positive mechanical stopping means. This eliminates the possibility of over-travel which can cause problems for many applications.
For fast actuating systems, the Freewheeling action gently dissipates motor and drive train momentum. This means very little shock is reflected back through the drive train, which is a common problem with the hard mechanical stops of conventional Ball Screw and power screw drives.
In addition to positive stroke control, Freewheeling Ball Screws offer high efficiency and long service life. All parts are made of quality carbon or alloy steel, and all wear surfaces are hardened for extended service life. Since ball return circuitry is not required, these units are very compact in design having convenient, round profiles with no protrusions needed for ball returns.
Freewheeling Ball Screws exhibit what is called an effective lead. The lead varies from the actual screw lead because of the planetary action of the balls as they rotate around the screw and nut. The effective lead varies slightly with load and, because of this, Freewheeling Ball Screws should not be used for positioning applications that use rotation for feedback, such as back gages on press brakes, stepper motor drives, or manual crank drives with handle dials.
The leads listed in Table 4 are nominal leads and apply to lightly loaded screws only. The effective lead will vary 10% depending upon the magnitude and direction of the load. If the load is in the same direction as the travel, the effective lead will be larger (i.e. faster) than the nominal lead. If the load opposes the direction of travel, the effective lead will be smaller than the nominal. In other words, it will take slightly more revolutions to raise a load than to lower a load using a Freewheeling Ball Screw.
Electrical switching devices and mechanical shifting mechanisms which require rapid travel rates and abrupt stops are ideal for Freewheeling type Ball Screws. Other applications include valves, bed and chair actuators, table drives, trim tab actuators, welding and cutting machine drives and virtually any motion requiring positive mechanical stops.
Freewheeling Ball Screws are traction type drives (see Effective Lead). The nut consists of annular grooves in place of the helical threads used in conventional nuts. Without some friction to maintain the grip between the screw, balls and nut, the nut will not advance along the screw shaft.
To maintain the minimum friction necessary to drive, lubricants which promote low sliding friction (i.e. grease with moly-disulfide additives) should never be applied to Freewheeling Ball Screws. This is especially important for applications where the load reverses from tension to compression over its stroke in mechanisms such as bi-stable flip flop switches. At zero load, the Ball Screw can freewheel in mid stroke if the wrong grease is used. Contact Roton Application Engineering for recommendations on lubricants for your installation.
The stroke length is determined by the location of the stop pins and is equal to the pin to pin dimension less the ball nut offset length (see Figure 5).
In order to allow for economical drilling of the stop pin holes, the pin to pin dimensions must be multiples of the pitch increments listed in Table 25.
(dia. x Lead)
|5/16 x .125||.100|
|9/16 x .083||.143|
|5/8 x .188||.167|
|3/4 x .250||.222|
|1 x .280||.250|