Double-rotor spindle motor

Abstract

The utility model discloses a double-rotor spindle motor, which comprises a front motor and a rear motor, and the front motor and the rear motor are coaxially butted along the axial direction through a middle cover. The rear end of the front motor rotating shaft is connected with the middle cover through a first bearing, and the front end of the front motor rotating shaft extends out of the front cover and is supported through a second bearing; the rear motor rotating shaft is coaxially nested in an inner cavity of the front motor rotating shaft, the front end of the rear motor rotating shaft extends out of the front motor rotating shaft, a clearance fit sliding sleeve is arranged between the front end of the rear motor rotating shaft and the front motor rotating shaft, the rear motor rotating shaft is connected with the middle cover and the rear cover through cylindrical roller bearings, and lengthened inner rings of the cylindrical roller bearings are fixedly mounted on the rear motor rotating shaft; the magnetic coupling feeding mechanism can drive the rotating shaft to synchronously move in the axial direction when the rotating shaft of the rear motor rotates; the front motor and the rear motor can independently control the rotating speed and the rotating direction of the rotating shafts, combined machining is supported, and the feeding mechanism is simple, convenient and small in size.

Description

Technical Field

[0001] This utility model relates to the field of spindle motor technology, specifically to a dual-rotor spindle motor. Background Technology

[0002] A spindle motor is a special type of motor designed for high-precision, high-speed rotary motion, and is widely used in CNC machine tools, precision machining, semiconductor equipment, and other fields. It is a direct drive or transmission device that converts electrical energy into high-speed rotary mechanical energy, directly driving loads such as cutting tools and grinding wheels.

[0003] Conventional electric spindles and motors have fixed output shafts and simple functions. In the field of composite machining, multiple spindle motors are required, resulting in large machine structure and weight, and high manufacturing costs.

[0004] In the fields of machining and precision manufacturing, the axial feed mechanism of the spindle is a core module that determines the compactness, dynamic response, and cost-effectiveness of the equipment. Traditional mechanical or electric spindle feed schemes mainly rely on slide screw mechanisms or bearing transition connections. However, their technical framework suffers from the following systemic bottlenecks: 1. Mechanical redundancy and space inefficiency: Slide screws require multiple layers of nested guide rails and support components, resulting in axial length redundancy (typically accounting for 30%-50% of the total spindle length), leading to a bulky overall size. While bearing transition structures can alleviate rigid impacts, they require additional couplings, flanges, and other connecting components, further encroaching on mechanical space. 2. Limited dynamic performance: Screw drives suffer from significant frictional losses and backlash, easily exhibiting creep phenomena at high speeds (accuracy loss reaches ±5μm at critical speeds), and are highly dependent on lubrication, prone to jamming under high-temperature conditions. The elastic deformation of the bearing transition mechanism introduces phase delay, affecting the real-time performance of micron-level feed. 3. Cost and adaptability conflict: The manufacturing cost of high-precision ball screws and customized bearing components accounts for up to 40%, but their drive source is only compatible with rotary motors or hydraulic motors, making it difficult to be compatible with new power forms such as pneumatic and linear motors, which restricts the modular expansion capability of the equipment. Utility Model Content

[0005] In order to overcome at least one of the technical problems existing in the prior art, this utility model provides a dual-rotor spindle motor in which tools can be installed at the front ends of the front motor shaft and the rear motor shaft for processing. The front motor and the rear motor can independently control the speed and direction of their respective shafts, supporting composite processing. The feeding mechanism is simple and convenient and has a small size.

[0006] A dual-rotor spindle motor includes a front motor and a rear motor. Both the front and rear motors include a stator, a front cover mounted at the front end of the front motor, and a rear cover mounted at the rear end of the rear motor. The front and rear motors are coaxially connected via a middle cover, forming a dual-rotor independent drive structure. The front motor shaft is a hollow shaft that extends axially through the center. Its rear end is connected to the middle cover via a first bearing, and its front end extends beyond the front cover and is supported by a second bearing. The rear motor shaft is coaxially nested within the cavity of the front motor shaft. Its front end extends beyond the front motor shaft and has a clearance-fitted sleeve between it and the front motor shaft. The rear motor shaft is connected to the middle and rear covers via cylindrical roller bearings, with an extended inner ring of the cylindrical roller bearing fixedly mounted on the rear motor shaft. Tool holders can be simultaneously mounted at the front ends of both the front and rear motor shafts. The front and rear motors can independently control the speed and direction of their respective shafts; it also includes a magnetic coupling feed mechanism that can drive the rear motor shaft to move synchronously along the axial direction when the rear motor shaft rotates; the magnetic coupling feed mechanism includes a feed shaft coaxially arranged with the rear motor shaft, and ball grooves are correspondingly opened on the end faces of the rear motor shaft and the feed shaft near the end, with a ball placed in each ball groove; a pressure cap is fitted on the outer wall of the rear motor shaft and the feed shaft near the end, and the two pressure caps are kept in a non-contact state; each pressure cap has a ball hole that runs through the axial direction, and the ball hole is positioned corresponding to the ball groove. The pressure cap restricts the ball in the area between the ball groove and the ball hole, and part of the outer wall of the two balls protrudes out of the ball hole and contacts each other; magnets with opposite poles are installed on the near ends of the two pressure caps, and the two magnets are kept in a non-contact state.

[0007] In some embodiments, axially protruding sleeve shafts are correspondingly provided on the adjacent end faces of the two tool holders, and an axially expandable bellows is sleeved between the two sleeve shafts. The sleeve shafts and the bellows are rotatably connected, and the axial pre-compression of the bellows is 8%-12% of the total length, and axial tension is maintained by elastic restoring force.

[0008] In some embodiments, the outer walls of the rear motor shaft and the feed shaft near the end are provided with threads for threaded connection with the gland; at least one fastener is installed on the outer wall of the gland, and the fastener fixes the two glands to the rear motor shaft and the feed shaft in a radial direction.

[0009] In some embodiments, the magnet is in the shape of a ring, and the end faces of the two pressure caps are provided with axially protruding positioning shafts, and the magnet is sleeved on the positioning shafts; it also includes at least two regularly distributed locking members, which fix the two magnets in axial direction corresponding to the rear motor shaft and the feed shaft.

[0010] In some embodiments, a flange is installed around the magnetic coupling feed mechanism, one end of which is coaxially connected to the rear cover and the other end is coaxially connected to the cylinder, which is used to drive the feed shaft to move back and forth axially.

[0011] In some embodiments, the outer wall of the sliding sleeve is interference-fitted with the inner wall of the front motor shaft, and the sliding sleeve is slidably fitted with the rear motor shaft; the extended inner ring of each cylindrical roller bearing extends axially and is interference-fitted with the rear motor shaft. The axial length of the extended inner ring is at least twice the axial length of the outer ring. The extended inner ring can provide a longer rolling surface for the rollers, so that the rear motor shaft can effectively support and slide feed within the feed stroke range.

[0012] Additional aspects and advantages of this invention will continue to be set forth in the description which follows, and in part will be obvious from the description or may be learned by practice of this invention. Attached Figure Description

[0013] The above and / or additional aspects and advantages of this utility model will become apparent and readily understood from the description of the embodiments taken in conjunction with the following drawings, in which:

[0014] Figure 1 This is a frontal stereoscopic structural diagram of this application;

[0015] Figure 2 This is a rear-view stereoscopic structural diagram of this application;

[0016] Figure 3 This is a front structural diagram of this application;

[0017] Figure 4 yes Figure 3 A schematic diagram of the cross-sectional structure;

[0018] Figure 5 yes Figure 4 Enlarged view of point A in the middle;

[0019] Figure 6 yes Figure 4 Enlarged view of point B in the middle;

[0020] Figure 7 This is a front structural diagram of the magnetic coupling feed mechanism;

[0021] Figure 8 yes Figure 7 A schematic diagram of the cross-sectional structure;

[0022] Figure 9 yes Figure 7 A schematic diagram of the structure after removing the sphere;

[0023] Figure 10 This is a three-dimensional structural diagram of the magnetic coupling feed mechanism;

[0024] Figure 11 This is a cross-sectional structural diagram of the cylindrical roller bearing and the rear motor shaft;

[0025] Figure 12This is a three-dimensional structural diagram of the cylindrical roller bearing and the rear motor shaft.

[0026] Figure label:

[0027] Front motor 1, Rear motor 2, Stator 3, Middle cover 4, Front motor shaft 5, ...

[0028] 6. Bearing 1, 7. Second Bearing 2, 8. Rear Motor Shaft 3, 9. Sliding Sleeve, 10. Cylindrical Roller

[0029] Bearing 10, extended inner ring 100, outer ring 101, roller 102, front cover 11.

[0030] Tool holder 12, magnetic coupling feed mechanism 13, feed axis 14, ball groove

[0031] 15. Sphere; 16. Gland; 17. Ball hole; 18. Magnet; 19. Sleeve shaft; 20. Wave.

[0032] 21. Cord 22. Back cover 23. Thread 24. Fastener 25. Positioning shaft 26. Locking mechanism

[0033] Part 26, flange 27, cylinder 28. Detailed Implementation

[0034] The embodiments of this utility model are described in detail below. Examples of the embodiments are shown in the accompanying drawings, wherein the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout. The embodiments described below with reference to the accompanying drawings are exemplary and are only used to explain this utility model, and should not be construed as limiting this utility model.

[0035] In the description of this utility model, it should be understood that the directional descriptions, such as the terms "up," "down," "front," "back," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," and "outer," indicate the directional or positional relationship based on the directional or positional relationship shown in the accompanying drawings. They are only for the convenience of describing this utility model and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this utility model.

[0036] In the description of this utility model, "several" means one or more, "multiple" means two or more, "greater than," "less than," and "exceeding" are understood to exclude the stated number, while "above," "below," and "within" are understood to include the stated number. If "first" or "second" is used in the description, it is only for the purpose of distinguishing technical features and should not be construed as indicating or implying relative importance, or implicitly indicating the number of indicated technical features, or implicitly indicating the order of the indicated technical features.

[0037] In the description of this utility model, it should be noted that, unless otherwise explicitly specified and limited, the terms "installation," "connection," and "joining" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal connection of two components. Those skilled in the art can understand the specific meaning of the above terms in this utility model based on the specific circumstances.

[0038] Reference Figures 1-12 A dual-rotor spindle motor includes a front motor 1 and a rear motor 2. Both the front motor 1 and the rear motor 2 include a stator 3, a front cover 11 mounted at the front end of the front motor 1, and a rear cover 22 mounted at the rear end of the rear motor 2. The front motor 1 and the rear motor 2 are coaxially connected via a middle cover 4 to form a dual-rotor independent drive structure. The front motor shaft 5 is a hollow shaft that extends axially. Its rear end is connected to the middle cover 4 via a first bearing 6, and its front end extends outside the front cover 11 and is supported by a second bearing 7. The first bearing 6 and the second bearing 7 can be deep groove ball bearings or angular contact ball bearings. Cylindrical roller bearings, etc.; the rear motor shaft 8 is coaxially nested within the inner cavity of the front motor shaft 5, with its front end extending outside the front motor shaft 5 and a clearance-fitting sleeve 9 provided between it and the front motor shaft 5; the rear motor shaft 8 is connected to the middle cover 4 and the rear cover 22 via cylindrical roller bearings 10, and the extended inner ring 100 of the cylindrical roller bearings 10 is fixedly installed on the rear motor shaft 8; the front ends of the front motor shaft 5 and the rear motor shaft 8 can simultaneously install tool holders 12; the front motor 1 and the rear motor 2 can independently control the speed and direction of their respective shafts; it also includes magnetic coupling... The feeding mechanism 13 can drive the rear motor shaft 8 to move synchronously along the axial direction when the rear motor shaft 8 rotates. The magnetic coupling feeding mechanism 13 includes a feeding shaft 14 coaxially arranged with the rear motor shaft 8. Ball grooves 15 are correspondingly opened on the end faces of the rear motor shaft 8 and the feeding shaft 14 near the end. A ball 16 is placed in each ball groove 15. A pressure cap 17 is fitted on the outer wall of the rear motor shaft 8 and the feeding shaft 14 near the end. The two pressure caps 17 are kept in a non-contact state. A ball hole 18 is opened on each pressure cap 17 and is axially penetrating. The ball hole 18 is positioned corresponding to the ball groove 15. The pressure cap 17 restricts the spheres 16 in the area between the ball groove 15 and the ball hole 18, and part of the outer wall of the two spheres 16 protrudes out of the ball hole 18 and contacts each other; the double ball contact forms a physical limit, suppressing the radial displacement of the magnet 19 caused by vibration, which can eliminate the response hysteresis of magnetic coupling and realize zero delay in power transmission. The contact friction damping absorbs high-frequency vibration energy, and even if the magnet 19 is accidentally demagnetized, power transmission can still be maintained through the ball contact; magnets 19 with opposite poles are installed at the near ends of the two pressure caps 17, and the two magnets 19 are kept in a non-contact state.

[0039] The hollow front motor shaft 5 and rear motor shaft 8 are dynamically coupled through the sliding sleeve 9. The hollow shaft nesting structure reduces the axial dimension by more than 40%, making it suitable for installation in confined spaces. The two shafts can operate independently at different speeds and in different directions, or they can rotate at the same speed and in the same direction. The front motor 1 can drive a high-torque grinding wheel for rough machining through the front motor shaft 5. The rear motor shaft 8 of the rear motor 2 can synchronously drive a precision polishing wheel for finishing. The reverse thermal deformation generated by the differential speed of the two rotors can offset more than 60% of the axial elongation. The bellows 21 provides a seal to prevent chips and ensures that the two tools work synchronously. The magnetic coupling mechanism has a lifespan of 2 million cycles, which is 5 times longer than that of a ball screw.

[0040] The feed shaft 14 can be driven by any mechanism capable of axial forward and backward movement, such as a cylinder, hydraulic cylinder, electric device, or lead screw, and can rotate or not. Two spheres are installed at the center, and two pressure caps fix the two spheres to the rear motor shaft 8 and the feed shaft 14. Partial areas of the two spheres abut each other, which can reduce the friction area and reduce the rotational resistance of the rear motor shaft 8. The magnets have strong magnetic force, and the end faces of the two magnets do not contact, maintaining a small distance, and the magnetic polarities are opposite. This ensures propulsion during the rotation of the rear motor shaft 8. When the feed shaft 14 moves forward, the rear motor shaft 8 moves forward synchronously; when the feed shaft 14 moves backward, the rear motor shaft 8 moves backward synchronously. The structural design of the two spheres and the two magnets with opposite polarities ensures that the rear motor shaft 8 moves backward synchronously, achieving seamless forward and backward movement during rotation.

[0041] This application solves the problem that a single motor cannot complete composite processing synchronously, for example, rough grinding and fine grinding need to be carried out in separate steps; it eliminates the repeated positioning error caused by switching between multiple processes, actively compensates for thermal deformation through the differential rotation of dual rotors, and achieves high-precision axial displacement through magnetic coupling feed mechanism, replacing the traditional ball screw.

[0042] In some embodiments, axially protruding sleeve shafts 20 are correspondingly provided on the adjacent end faces of the two tool holders 12, and an axially expandable bellows 21 is sleeved between the two sleeve shafts 20. The sleeve shafts 20 and the bellows 21 are rotatably connected. The axial pre-compression of the bellows 21 is 8%-12% of its total length, and the axial tension is maintained by the elastic restoring force. The composite structure of the metal layer (such as 304 stainless steel) and the rubber layer (such as fluororubber) of the bellows 21 provides good dynamic sealing. The 8%-12% pre-compression can balance the attenuation of elastic force.

[0043] In some embodiments, the outer walls of the rear motor shaft 8 and the feed shaft 14 near their ends are provided with threads 23 for threaded connection with the pressure cap 17; at least one fastener 24, preferably a non-removing screw, is installed on the outer wall of the pressure cap 17. The fastener 24 fixes the two pressure caps 17 to the rear motor shaft 8 and the feed shaft 14 in the radial direction to prevent loosening and to strengthen the fixing effect; the threads 23 achieve axial pre-tightening between the pressure cap 17 and the shaft 8 and the feed shaft 14, and the fastener 24 applies a locking force of >500N in the radial direction to eliminate the risk of thread loosening. The threads 23 bear 70% of the axial force, and the fastener 24 bears 30% of the radial force, optimizing the stress distribution. The pressure cap 17 can be replaced simply by loosening the fastener 24.

[0044] In some embodiments, the magnet 19 is ring-shaped, and the end faces of the two pressure caps 17 are provided with axially protruding positioning shafts 25, on which the magnet 19 is sleeved. It also includes at least two regularly distributed locking elements 26, preferably anti-removal screws, which axially fix the two magnets 19 to the rear motor shaft 8 and the feed shaft 14. The feed shaft 14 is the driving element, which can be driven axially by a cylinder 28, a linear motor, a hydraulic cylinder, or a manual screw. The rear motor shaft 8 is the driven element, passively following the magnets 19 through the opposite pole attraction (NS poles). The positioning shaft 25 provides radial positioning, and the locking elements 26 address axial displacement, improving reliability compared to a single fixing method. At a speed of 20,000 rpm, the radial runout of the magnet 19 is controlled within 0.005 mm, exhibiting good dynamic balance. The magnet 19 and the positioning shaft 25 form a standardized unit, supporting rapid adaptation to different power models.

[0045] In some embodiments, a flange 27 is installed around the magnetically coupled feed mechanism 13. One end of the flange is coaxially connected to the rear cover 22, and the other end is coaxially connected to a cylinder 28. The cylinder 28 is used to drive the feed shaft 14 to move back and forth axially. Driven by the cylinder 28, the pneumatic transmission response time is <10ms, which is 5-10 times faster than a servo motor. It has good anti-pollution capabilities, and its axial length is 40% shorter than that of a ball screw module, making it more compact. It is directly coaxially integrated through the flange 27, saving lateral space.

[0046] In some embodiments, the outer wall of the sliding sleeve 9 is interference-fitted with the inner wall of the front motor shaft 5, and the sliding sleeve 9 is slidably sleeved with the rear motor shaft 8; the extended inner rings 100 of the two cylindrical roller bearings 10 extend axially and are interference-fitted with the rear motor shaft 8. The axial length of the extended inner ring 100 is at least twice the axial length of the outer ring 101. The extended inner ring 100 can provide a longer rolling surface for the roller 102. The extended inner ring 100 is equivalent to a sliding sleeve. The extended inner ring 100 and the motor shaft 8 assembly can move and rotate axially relative to the roller 102, so that the rear motor shaft can effectively support and slide feed within the feed stroke range.

[0047] The outer wall of the sliding sleeve 9 is interference-fitted with the inner wall of the front motor shaft 5, forming a rigid torque transmission interface to avoid energy loss caused by relative sliding. The sliding sleeve 9 is slidably sleeved with the rear motor shaft 8, allowing axial thermal expansion displacement, while achieving precise radial positioning through the extended inner ring 100 of the cylindrical roller bearing 10. The extended inner ring 100 (length ≥ 2 times that of the outer ring 101) provides an extended rolling surface for the roller 102, reducing the risk of roller misalignment (especially at high speeds), and suppressing radial vibration of the rear motor shaft 8 through the interference fit. The sliding sleeve design compensates for the difference in axial thermal expansion during motor operation, while the interference-fitted bearing inner ring absorbs radial thermal stress through metal elastic deformation. This solves the problem that traditional equal-length inner ring bearings are prone to gyroscopic effects caused by short-distance roller guidance at high speeds (>8000rpm), leading to excessive vibration.

[0048] In some embodiments, the ball groove 15 is a circular cavity, the ball 16 is made of quenched alloy steel, and the magnet 19 is a neodymium iron boron permanent magnet. The circular cavity ball groove 15 can better fit the shape of the ball 16. The hardness of the quenched alloy steel reaches 60-65 HRC (Rockwell hardness), which is 3 times higher than that of untreated steel. It has the characteristics of ultra-high hardness, significantly extended fatigue life, and outstanding compressive strength. The neodymium iron boron magnet 19 (N52 grade) generates a 1.4T magnetic field, which enhances the axial pressure through the attraction of opposite poles. The sliding sleeve 9 is a copper sleeve, which has the advantages of self-lubrication, low friction, corrosion resistance, good thermal conductivity, vibration reduction and noise reduction. The extended inner ring is a steel sleeve, which has the advantages of high strength, wear resistance, low cost, and strong load-bearing capacity.

[0049] The technical features of the above embodiments can be combined arbitrarily. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification. Although embodiments of the present invention have been shown and described, those skilled in the art will understand that various changes, modifications, substitutions and variations can be made to these embodiments without departing from the principles and spirit of the present invention. The scope of the present invention is defined by the claims and their equivalents.

Claims

1. A dual-rotor spindle motor, comprising a front motor and a rear motor, both the front and rear motors including a stator, a front cover mounted at the front end of the front motor, and a rear cover mounted at the rear end of the rear motor, characterized in that: The front and rear motors are coaxially connected via a middle cover, forming a dual-rotor independent drive structure. The front motor shaft is a hollow shaft that runs axially through the center. Its rear end is connected to the middle cover via a first bearing, and its front end extends outside the front cover and is supported by a second bearing. The rear motor shaft is coaxially nested within the cavity of the front motor shaft. Its front end extends outside the front motor shaft and has a clearance-fitting sleeve between it and the front motor shaft. The rear motor shaft is connected to both the middle and rear covers via cylindrical roller bearings, with the extended inner ring of the cylindrical roller bearings fixedly mounted on the rear motor shaft. Tool holders can be mounted simultaneously on the front ends of both the front and rear motor shafts. The front and rear motors can independently control the speed and direction of their respective shafts. Magnetic force is also included. The coupling feed mechanism can drive the rear motor shaft to move synchronously along the axial direction when the rear motor shaft rotates. The magnetic coupling feed mechanism includes a feed shaft coaxially arranged with the rear motor shaft. Ball grooves are correspondingly opened on the end faces of the rear motor shaft and the feed shaft near the end, and a ball is placed in each ball groove. A pressure cap is fitted on the outer wall of the rear motor shaft and the feed shaft near the end, and the two pressure caps are kept in a non-contact state. A ball hole is opened on each pressure cap along the axial direction. The ball hole is positioned corresponding to the ball groove. The pressure cap restricts the ball in the area between the ball groove and the ball hole, and part of the outer wall of the two balls protrudes out of the ball hole and contacts each other. A magnet with opposite poles is installed on the near end of each of the two pressure caps, and the two magnets are kept in a non-contact state.

2. The dual-rotor spindle motor as described in claim 1, characterized in that: in Two tool holders are provided with axially protruding sleeve shafts on their adjacent end faces. A bellows that can expand and contract axially is sleeved between the two sleeve shafts. The sleeve shafts and the bellows are rotatably connected. The axial pre-compression of the bellows is 8%-12% of the total length, and the axial tension is maintained by elastic restoring force.

3. The dual-rotor spindle motor as described in claim 1 or 2, characterized in that: Both the rear motor shaft and the feed shaft have threads on their outer walls near the end for connection with the gland threads; at least one fastener is installed on the outer wall of the gland, and the fastener fixes the two glands to the rear motor shaft and the feed shaft in the radial direction.

4. The dual-rotor spindle motor as described in claim 3, characterized in that: The magnet is in the shape of a ring, and the end faces of the two pressure caps are provided with axially protruding positioning shafts. The magnet is sleeved on the positioning shafts. It also includes at least two regularly distributed locking members, which fix the two magnets to the rear motor shaft and the feed shaft along the axial direction.

5. The dual-rotor spindle motor as described in claim 4, characterized in that: in The magnetic coupling feed mechanism is equipped with a flange on its periphery. One end of the flange is coaxially connected to the rear cover, and the other end is coaxially connected to the cylinder. The cylinder is used to drive the feed shaft to move back and forth along the axial direction.

6. The dual-rotor spindle motor as described in claim 1 or 2, characterized in that: The outer wall of the sliding sleeve is interference-fitted with the inner wall of the front motor shaft, and the sliding sleeve is slidably fitted with the rear motor shaft; the extended inner ring of each cylindrical roller bearing extends axially and is interference-fitted with the rear motor shaft. The axial length of the extended inner ring is at least twice the axial length of the outer ring. The extended inner ring can provide a longer rolling surface for the rollers, so that the rear motor shaft can effectively support and slide feed within the feed stroke range.

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