DESC:FIELD OF THE INVENTION
The invention relates to controlling axial preload on ball bearings. More particularly, the present invention relates to controlling the axial preloading on the ball bearings in a can stack stepper digital linear actuator (DLA).

BACKGROUND OF THE INVENTION
The can stack stepper DLA has an over Molded permanent magnet rotor with internal threads for transferring the rotary motion to linear motion through a leadscrew of DLA.

In the DLA, the ball bearings are preloaded using a wave spring washer or helical spring washer to have zero axial play of rotor and bearing assembly when DLA is in operation. The preloading of the ball bearings is necessary for efficient operation of the system. Preloading removes or controls the internal clearance of the ball bearings. Further, preloading is required as degree of internal clearance within a bearing affects factors such as noise, vibration, and heat build-up. Thus, proper application of preloading is required for controlling the above mentioned problems. Controlled and exact application of preloading controls radial and axial play, provide system rigidity, prevent backlash of the rotor-ball bearing sub-assembly, and reduces non-repetitive run-out and increases the life of the ball bearings.

In the existing prior art, the compression of the spring is determined by the gap between the front ball bearing and the wall of the plastic bobbin housing (cap). This gap may vary due to the tolerances on the bearings, over-molded rotor, front and rear plastic bobbin housings (caps), the inner and outer claw pole cups. This gap variation has effect on the preload force which varies hugely due to the higher stiffness of the washer required. To reduce this variation, the tolerances of all the above components needs to be tightened which incurs additional component cost or frequent replacement of toolings. Also, due to this huge variation in the preload force, the overall efficiency of the DLA is reduced as additional torque is required to overcome the extra friction on the rotating assembly.

Thus, there is a need for a can stack stepper DLA which provides better control on the axial preload force on the ball bearings without tightening the tolerances of the parts and thus not increasing the cost.

OBJECTS OF THE INVENTION
It is an object of the invention to provide a can stack stepper DLA which provides controlled axial preloading of the ball bearings.

It is another object of the invention to provide a can stack stepper DLA in which axial preloading of the ball bearings is achieved without increasing tolerance of components.

It is still another object of the invention to provide a can stack stepper DLA in which axial preloading of the ball bearings is controlled economically.

It is still another object of the invention to provide a can stack stepper DLA in which axial preloading of the ball bearings is controlled efficiently.

It is yet another object of the invention to provide a can stack stepper DLA in which axial preloading of the ball bearings reduces noise in the motor.

It is yet another object of the invention to provide a can stack stepper DLA in which axial preloading of the ball bearings reduces heat built-up in the motor.

It is yet another object of the invention to provide a can stack stepper DLA in which axial preloading of the ball bearings reduces vibration in the motor.

It is yet another object of the invention to provide a can stack stepper DLA in which axial preloading of the ball bearings provides system rigidity (no axial play of leadscrew and rotating assembly during running).

It is yet another object of the invention to provide a can stack stepper DLA in which axial preloading of the ball bearings increases the life of the motor.

SUMMARY OF THE INVENTION
The present invention discloses an improved can stack stepper DLA. A particular advantage of this new DLA is that preloading of the ball bearings is achieved by pressing or gluing or welding/ fixing one of the caps i.e. front or rear caps to a pre-determined preload. The can stack stepper DLA comprises a rotating assembly, a front and a rear ball bearings, a front cap, a rear cap, a helical or Wave spring washer for preloading the ball bearings wherein compression of the preloading washer is controlled by pressing or gluing or welding/ fixing either of the front and rear caps to a pre-determined preload.

BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be better understood from the following detailed description of the preferred embodiments thereof, taken in conjunction with the accompanying drawing, wherein like reference numerals refer to like parts, in which:
Figure 1 illustrates structure of the can stack stepper DLA according to an embodiment of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
The detailed description set forth below in connection with the appended drawing is intended as a description of various embodiments of the present invention and is not intended to represent the only embodiments in which the present invention may be practiced. Each embodiment described in this disclosure is provided merely as an example or illustration of the present invention, and should not necessarily be construed as preferred or advantageous over other embodiments. The detailed description includes specific details for the purpose of providing a thorough understanding of the present invention. However, it will be apparent to those skilled in the art that the present invention may be practiced without these specific details.
Figure 1 illustrates a can stack stepper DLA 100 according to an embodiment of the present invention. The DLA includes a front cap 101, a rear cap 102 and a stator assembly which houses a rotor assembly, two ball bearings and a leadscrew assembly 112. The stator assembly is formed around a plurality of claw pole plates 107 which act as multipole electromagnets leading the magnetic field in the stator assembly. Said pole plates are arranged in pairs wherein the claw pole plates are assembled in adjacent spaced apart pairs. The stator assembly further includes wells located between the respective pairs of pole plates wherein the wells have a coil assembly 108 for generating magnetic flux when the current is passed. The stator assembly also has a passage there-through for receiving a rotor assembly. The rotor assembly includes an over molded rotor 105 for housing a magnet and an internal thread for converting rotation of the magnet to linear movement of a leadscrew 113 in the leadscrew assembly 112. The over molded rotor 105 is supported by two ball bearings i.e. the front ball bearing 103 and the rear ball bearing 104 to support rotation of the rotor assembly relative to the stator assembly.
According to an embodiment of the invention, the ball bearings are configured to receive a preload washer 106 to provide stability to the system during operation. The preload washer 106 is placed between one or both of the caps (front cap 101 and rear cap 102) and its adjacent ball bearing (front ball bearing 103 and rear ball bearing 104). Preloading of the ball bearings is necessary to remove spaces/clearances, play of the rotor and bearing assembly and provide rigidity during operation. The preload washer may be a wave spring washer or helical spring washer to have zero axial play of the rotor and bearing assembly. According to another embodiment, a flat washer may be used instead of a preload washer to achieve zero axial play.
According to an embodiment of the invention, one/either of the caps i.e. front and rear caps are made in two parts. Thereafter, components are assembled and the compression of the preload spring washer (gap) is controlled by pressing/gluing/welding/fixing one of the caps to a pre-determined distance based on the preload force required. This allows the present invention to have a rigid rotor assembly with a flat washer when any application calls for a zero axial play even at higher axial loads. The compression of the spring washer 106 and the stroke length is controlled by pressing/gluing/welding/fixing either of the front and rear caps to a pre-determined distance thus providing the flexibility of using different compression levels for different motor assemblies. In the existing DLAs, the method of preloading has huge variations due to the varying gap from motor to motor for the preload washer. This restricts the DLA in having zero axial play option due to this gap variation. The present invention addresses this issue by having access and control to this gap by a separate front Cap 101 and/ or rear cap 102.
The separate front cap 101 or rear cap 102 allows control over the gap for the preload washer. Even when the dimensions of other components in the assembly may vary within their manufacturing tolerances, the gap may still be maintained as per requirement by adjusting the front or rear cap position inside the stator over-mold and fixing it with any joining process.
Since other modifications and changes to fit particular requirements and environments will be apparent to those skilled in the art, the invention is not considered limited as described by the present preferred embodiments which have been chosen for purposes of disclosure, and covers all changes and modifications which do not constitute departure from the spirit and scope of this invention.

,CLAIMS:We Claim:
1. A can stack stepper digital linear actuator comprising:
a front cap;
a rear cap;
a stator assembly formed around a plurality of claw pole plates acting as multipole electromagnets;
a rotor assembly housed in a passage in the said stator assembly, said rotor assembly having a rotor for housing a magnet and an internal thread; and
a pair of ball bearings and a leadscrew assembly wherein said ball bearings are preloaded.

2. The can stack stepper digital linear actuator as claimed in claim 1, wherein the ball bearings are preloaded by a wave spring washer.

3. The can stack stepper digital linear actuator as claimed in claim 1, wherein the ball bearings are preloaded by a helical spring washer.

4. The can stack stepper digital linear actuator as claimed in claims 1, 2 and 3, wherein preloading is controlled by pressing or gluing or welding or fixing of front cap or rear cap to a pre-determined distance.

5. The can stack stepper digital linear actuator as claimed in claim 1, wherein front and or rear caps are made in two parts.

6. The can stack stepper digital linear actuator as claimed in claims 1, 2 and 3, wherein the wave spring washer or helical spring washer is placed between the front cap and the front ball bearing.

7. The can stack stepper digital linear actuator as claimed in claims 1, 2 and 3, wherein the wave spring washer or helical spring washer is placed between the rear cap and the rear ball bearing.
Dated: 07.07.2016

(JAYANTA PAL)
IN/PA 172
REMFRY & SAGAR
ATTORNEY FOR THE APPLICANT [S]