Axial Clamping for Frameless Motor Assembly: Principles and Practice

Axial clamping is a mechanical retention method where a motor stator or rotor is secured by compressing it along the motor’s axis between two rigid parts. In practice, one housing component provides a shoulder (or “banking surface”) that the stator sits against, and a separate clamp ring or end bell is then bolted on top. Tightening the fasteners draws the two parts together, creating frictional contact that locks the stator stack in place. Proper radial alignment is ensured by a precise fit of the stator’s outer diameter in the housing (or the rotor’s inner diameter on the shaft), while axial position is controlled by the shoulder and clamp geometry. In other words, the stator is “sandwiched” axially: it banks against a fixed shoulder on one side, and a clamp ring (often a full 360° ring or several evenly spaced clamps) on the other applies uniform axial pressure.

Benefits of Clamping Over Adhesive Bonding

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Using axial clamping instead of adhesive bonding offers several practical advantages in motor assembly. No cure time or chemicals are required, which can greatly speed up the assembly process. With clamping, there is no need to apply epoxy or anaerobic adhesive and wait for it to cure, eliminating potential delays and the complexity of chemical handling. Moreover, an axially clamped assembly can be taken apart if needed: unlike a permanent adhesive joint, the stator and rotor can be disassembled for inspection or maintenance. This ease of disassembly is explicitly noted in practice – axial clamps “provide reliable stator positioning” while allowing “easy removal for maintenance.” Finally, clamping avoids the risk of adhesive degradation over time or outgassing, and it sidesteps any need to match housing materials for thermal expansion (a concern when using press-fit or shrink-fit designs). In short, axial clamping yields a robust, serviceable joint without chemical bonding steps.

Key Considerations: Alignment, Retention, and Thermal Performance

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When using axial clamping, several factors must be carefully addressed:

• Radial and Axial Alignment: Achieving precise alignment is critical. Radial alignment comes from a close fit (a line-to-line or transitional fit) between the stator’s outer diameter and the housing bore, or between the rotor ID and the shaft. Axial alignment is set by hard stops or shoulders in the housing or shaft. For example, a machined shoulder on one housing half can define the stator’s axial position while the clamp ring on the other side completes the sandwich. It is important that any hard stop or shoulder contacts only the motor’s aluminum backing ring (not the coils or magnets) to avoid damage.
  
  
• Retention Surfaces: The clamp must bear on the proper surfaces. For stators, most slotless designs have a thin steel back-iron ring sandwiched by aluminum end rings; the clamp should engage the rigid back-iron or aluminum ring, not the insulated windings. For rotors with Halbach magnet arrays, the clamp must engage the rotor’s carrying ring rather than the brittle magnets themselves. Use a full circular clamp ring or multiple evenly spaced clamps to distribute force uniformly around the stack. Mounting bolts (4–12 bolts equally spaced is common) should be torqued to spec so that the clamp exerts sufficient pressure without deforming the parts.
  
  
• Torque and Load Requirements: Assess the motor’s torque and shock requirements. Many guidelines suggest axial clamping is well suited for low-to-moderate torque applications where periodic disassembly might be needed. For very high torque or heavy vibration, supplementary methods (adhesive, shrink-fit, or additional clamps) may be advisable. If high clamping force is needed, you can increase the clamp surface area or number of bolts to raise total holding power.
  
  
• Thermal Performance: Thermal management should not be overlooked. Any air gap between the stator and housing increases the thermal resistance. It is common practice to fill the small clearance between the stator OD and housing bore with a thermally conductive interface material, for instance, silicone-based thermal grease or a thin film adhesive, to improve heat transfer. Also ensure the clamp ring’s contact surfaces are clean and flat for good heat transfer. Overall, providing good thermal paths (with grease or thermally conductive adhesive in the gap) can significantly improve motor cooling.
  
  
• Component Protection: The clamp should not damage delicate parts. Only apply clamping forces through the robust aluminum/steel rings. Specifically, do not bear the clamp on magnets or windings. Ensure there is sufficient clearance between clamp features and the winding ends so that all axial load goes through the intended surfaces.
  
  

Best Practices for Applying Clamping Force Safely

To apply axial clamp force without damaging the motor:

• Force Path: Direct all clamp forces through the designated structural rings. Any hard stop or clamp contact should only touch the motor’s aluminum backing ring, not the coil potting or magnet faces. This prevents crushing or distorting the windings and magnets.
  
  
• Fastening: Use multiple bolts or clamps evenly spaced around the ring. Tighten bolts in an alternating pattern to apply even pressure. Follow the manufacturer’s torque specifications. For example, in one slim-torque frame example, four M3 screws were used torqued to about 1 N·m (ensure your design’s screws and torque match the motor’s size and torque).
  
  
• Anti-Loosening: Use locking features on clamp bolts. A lock washer, thread locker, or anaerobic adhesive on the bolts can prevent them from backing out under vibration. For instance, a lock washer or adhesive can be applied to keep clamp screws from loosening.
  
  
• Surface Cleanliness: Keep clamping surfaces clean and dry. Oil, grease, or dust on the mating surfaces can reduce friction and effective clamping force. In particular, avoid contaminating the clamp faces with lubricant: design with a clearance so that no grease can migrate into the clamping surfaces.
  
  
• Design Clearances: The mechanical design should include an axial gap (clearance) behind the clamp ring when the stator is installed. This gap ensures the clamp ring only contacts the stator (and not another shoulder) as bolts are tightened. If the clamp ring could hit the housing before tightening, the stator wouldn’t be properly compressed.
  
  
• Component Protection: During assembly, handle the stator and rotor carefully. For example, always clean components (the stator OD, rotor ID, housing bores and clamping faces) with isopropyl alcohol on a lint-free cloth before assembly. This removes oils that might impair contact or thermal transfer.
  
  

Step-by-Step Assembly of the ST-130-27-Max Motor

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The following outlines one way to assemble the Alva ST-130-27-Max framed motor using axial clamping. (Refer to the specific mechanical drawings for exact part numbers and torque values.)

1. Prepare the Housing Halves. Insert the appropriate bearings into each of the two housing end-caps or frames. Clean all contact surfaces (the stator OD, housing bores, and clamp faces) with isopropyl alcohol and a lint-free cloth.
   
   
2. Apply Retaining Compound (Optional). On one housing half (the side where the stator’s terminal block will sit), apply a thin coat of an anaerobic retaining compound (e.g. Loctite 638) or thermal grease to the bore face. This can enhance friction and thermal contact; if preferred, a small amount of silicone thermal grease can be used instead.
   
   
3. Insert the Stator. Place the stator stack into the prepared housing half, pushing it axially until it seats against the housing’s internal shoulder. The stator’s outer aluminum ring will bank on this shoulder. Ensure the stator is oriented so its windings and terminals align correctly (e.g. terminal block facing the intended side).
   
   
4. Attach Rotor and Opposing Housing. Place the rotor (on its shaft) into the other housing half (which also has its bearing installed). Then mate the two halves: push the second housing half (with rotor) onto the shaft until it approaches the stator. In this step, the rotor will insert into the stator, and the second housing half will come together with the first. The clamping gap between stator and second housing will close slightly as they meet. Make sure the rotor-stator axial alignment is correct; machined shoulders or a flange on the shaft often ensure this.
   
   
5. Install Clamp Bolts. With the stator sandwiched between the two housing halves, insert clamp bolts through the second housing into the first housing (or clamp ring) around the circumference. For example, four M3 screws (evenly spaced at 90°) can be used. Tighten the bolts progressively and evenly in a criss-cross pattern to about the specified torque (the example assembly used ~1 N·m for M3 screws). This draws the two housing halves together, compressing the stator axially against the shoulder. Do not overtighten, and ensure the clamping force is applied uniformly through the stator’s backing ring.
   
   
6. Final Checks and Encoder Installation. After bolting, verify there is no play in the assembly and that the stator is firmly held. Check that the clamp ring has not bottomed out prematurely (confirm an intentional gap remained) and that the stator did not shift. Finally, install the motor’s encoder or end-bell as required. The motor is now assembled with an axial clamp mechanism securing the stator.
   
   

Throughout assembly, ensure all forces are directed through the designed structural rings (the aluminum/steel back-iron). This will protect the coils and magnets from stress. With careful attention to alignment, cleanliness, and even clamping force, axial clamping provides a reliable, adhesive-free motor assembly that is serviceable and thermally efficient.

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