High-Volume HVAC Part Isolation and Torque Specifications

HVAC Motor Isolation Requirements

An HVAC OEM in India sent us a new project. They needed a custom rubber fan motor hub. First, this rubber mount holds a heavy external rotor motor. The part must run well in extreme heat and cold.

It must survive temperatures from -40C to +100C. So, we knew the final machine would live outdoors. Bad outdoor weather breaks down weak rubber very fast. Next, the mechanical rules were very strict.

The main mount must handle a strong pull force. It needs to survive 1000 N of tensile force without tearing. Also, the twisting stress is very high. The mount must handle 12.0 N.m of turning torque.

This project requires a massive part count. First, the buyer needs 800,000 to 900,000 main threaded sleeves each year. Next, they want 200,000 to 300,000 secondary bases. Making this many parts takes careful factory work.

Every single rubber fan motor hub must be perfect. We cannot allow any bad batches. Furthermore, the drawings ban hexavalent chromium. So, the metal plating must follow strict RoHS rules.

Finally, the mount must block deep vibrations. It joins the spinning motor to the metal frame. We test fan airflow to AMCA 210-16 methods. Good mounts keep the whole HVAC system quiet.

Engineering the Rubber Fan Motor Hub Bond

We had to bond thick rubber to hard metal. This bond cannot break under high twisting force. First, we looked at standard EPDM rubber. EPDM is cheap.

It is also easy to mold. However, EPDM cannot hold a strong bond under 12.0 N.m of torque. The rubber peels away from the metal insert. So, we rejected EPDM right away.

Instead, we chose a custom chloroprene rubber. People often call this material Neoprene. The rubber fan motor hub needs chloroprene to survive heavy loads.

Formulating the Custom Rubber

Chloroprene stops bad ozone damage. It also bonds very well to treated steel. First, we use SPCC JIS steel for the core insert. Next, we plate this steel with zinc.

The zinc layer is an EP-FE/Zn coat. It must be at least 5 micrometers thick. Then, our workers apply a special metal glue. They paint this primer directly on the steel.

Next, the part goes into the hot rubber mold. So, the chemical glue creates a forever bond. In fact, we test the parts until they break. The rubber must rip in half.

The bond to the metal must never fail. Finally, any extra rubber flash cannot exceed 0.5 mm.

Controlling Hardness and Friction

The two parts need different stiffness levels. First, the main sleeve uses a Shore A hardness of 55 +/- 5. This makes the rubber firm. It keeps the motor shaft perfectly straight.

Also, the flat surface must be very smooth. Surface bumps on Side A must stay under 0.25 mm. Side B bumps must stay under 0.3 mm. By contrast, the secondary base uses much softer rubber.

We use a Shore A 40-45 mix here. This soft base eats low-frequency hums. Next, the base has a 6 mm POM hole in the middle. POM is a hard, slick plastic.

So, the metal motor shaft rubs against smooth POM. It will not tear the soft rubber.

The Cost of High Reliability

This strong design costs more money to make. First, chloroprene takes longer to bake in the mold. It cooks slower than standard cheap plastics. Next, painting the metal glue takes time.

It adds a slow step to the factory line. Therefore, it takes more time to build each part. This raises the exact price of the rubber fan motor hub. Even so, the buyer liked our plan.

They accepted the higher price right away. In short, spending a little more prevents broken motors in the field.

Technical Specifications

Clearly, the Shore A hardness numbers matter most here. A hardness of 55 +/- 5 keeps the heavy motor centered. Next, the 1000 N pull rating is vital. It stops the part from ripping apart during a fast start. So, the rubber fan motor hub stays safe and quiet. Finally, the strict plating rules stop deep rust.

Reference: AMCA 210 covers the test method behind these figures.

Technical Documentation

Here are the official project files. You can read the exact engineering rules.

View Component Specification Document 1
First, this PDF shows the sizes for the threaded sleeve.
View Component Specification Document 2
Next, this PDF shows the soft rubber rules for the secondary base.

Furthermore, this image shows the inside of the rubber fan motor hub.
View Component Specification Document 4
Finally, this PDF lists the required zinc plating rules.

How to Specify High-Stress Motor Isolators

Engineers often guess the wrong twisting force for big HVAC mounts. If you need a heavy-duty mount, follow these rules.

• First, find the exact stalling torque. Measure the worst-case starting twist of your motor. Then, add a safety buffer. Do not just ask for a basic rubber part. Instead, name the exact Newton-meter load it must hold.
• Next, ask for a break test. Tell your factory to pull the part until it dies. The rubber must tear first. The glue must never fail. So, this proves the factory baked the part correctly.
• Furthermore, pick the right rubber stiffness. Use a soft Shore A 40 mix for low hums. Use a hard Shore A 55 mix to stop wobbles.
• Finally, watch out for rubbing parts. If a metal shaft spins against the mount, add a hard plastic insert. A POM insert works very well. Otherwise, the spinning shaft will chew through the soft rubber. We make our own rubber molds, so we check these tight fits easily.

Related: Fan Accessories.

Common Engineering Questions

Browse our axial fans range, the AC axial fans section, or EC axial fans.