How to Mount a Frameless Torque Motor Using a Thin Bond Line Concept

This article introduces the thin bond line mounting method, demonstrated using Alva’s SlimTorq™ motor with 130 mm outer diameter, in the Max version (STM-130-27-M). This method is one of several mounting options that are covered by our instructional video series. You can also watch the full video for a complete walkthrough of the process.

May 14, 2025

The correct mounting of a frameless motor is essential to ensuring optimal performance, durability, and thermal efficiency - especially in applications that demand compact, high-performance motor integration. At Alva, we design high-torque slotless motors for a wide range of use cases, including gimbal motors, robotics, and aerospace systems.

What Is a Thin Bond Line?

The concept of thin bond line mounting involves creating a snug fit between:

The outer diameter (OD) of the stator and the motor housing, or
The inner diameter (ID) of the rotor and the shaft.

The fit should be a close clearance or transition fit. This ensures good radial alignment without the need for mechanical fasteners or complicated fixtures during curing. The key to the thin bond line method lies in the use of retaining compounds, such as Loctite 638 or 648, which are specially formulated adhesives for bonding cylindrical, close-fitting components.

Radial and Axial Alignment

In a well-executed thin bond line assembly, the mounting typically has two main important advantages:

Radial alignment is achieved through the tight fit between components.
Axial alignment is maintained using a hard stop feature, either on the stator housing or the rotor shaft.

It’s important that the hard stop has contact only with the outer aluminum ring of the stator or the inner aluminum ring of the rotor. Any force applied during assembly should be directed through these rings to prevent damage to other parts of the motor (e.g. the potting of the windings).

Surface Preparation Best Practices

Good surface preparation is essential for reliable adhesion and long-term motor performance. At Alva, we recommend following these surface prep steps:

For anodized aluminum (e.g. stator OD):

Clean with isopropanol alcohol using a lint-free cloth.
Allow the surface to dry completely.

For bare aluminum (e.g. rotor ID):

Clean first with isopropanol to remove any oils or contaminants.
Roughen the surface using 80-grit sandpaper to improve adhesion.
Clean again with isopropanol to remove sanding residue, and let it dry.

Note: Cleaning before sanding helps prevent oil from being pushed into the metal surface.

Proper surface preparation enhances both the adhesive bond strength and the thermal performance of the motor. Adhesives are significantly more thermally conductive than air, so filling the stator-to-housing gap helps eliminate air pockets and improves heat transfer.

Step-by-Step Mounting: Rotor to Shaft

In our demonstration using the SlimTorq™ motor, we apply the thin bond line method to mount the rotor to the shaft:

Initial cleaning
Clean both the shaft OD and rotor ID with isopropanol and a lint-free cloth.

Surface roughening
Use 80-grit sandpaper to create a rough surface profile.
Final cleaning
Clean again with isopropanol to remove dust and allow it to dry.
Adhesive application
Apply a thin layer of Loctite 638 to both surfaces.
Assembly
Gently press the rotor onto the shaft using a manual press and adapter.
Finishing step
Wipe away any excess adhesive.

The result is a clean, secure bond that ensures reliable mechanical and thermal contact between components. The rotor is now firmly in place and ready for the next steps in the frameless torque motor assembly.

Why Use a Thin Bond Line?

The thin bond line method is ideal for engineers and integrators working with frameless slotless motors in applications like:

Gimbal motors
Robotic joints
Aerospace systems
Precision actuators

This method ensures:

High alignment precision
Minimal thermal resistance
Strong, stable mechanical bonding
No need for mechanical fasteners or heavy fixtures

Watch the Full Video

Watch the full step-by-step demonstration in our video tutorial here.

Explore Frameless Motors from Alva

At Alva, we design and manufacture slotless, frameless torque motors using our proprietary FiberPrinting™ technology. Our SlimTorq™ series delivers high torque density, large through-holes, and zero cogging - perfect for dynamic, lightweight systems.

Use our Product Selector Tool to find the right motor for your application.

Have questions? Visit www.alvaindustries.com or get in touch with our engineering team.