Need a High-Performance Fastener Alternative?  Send Us Your Specs

Need a High-Performance Fastener Alternative? Send Us Your Specs

Case Study

From Press-Fit to Field-Serviceable: Redesigning a PTU Bearing Assembly

HVAC Packaged Thermal Unit Manufacturer

Press fits trapped a PTU’s bearing inside the casement, where heat and moisture caused early failure. Tolerance rings moved it outside, and made field repair possible.

Root Cause Analysis

The CHALLENGE with Interference Fits at Temperature

Packaged thermal units (PTUs) are self-contained air conditioners and heat pumps. They often include a blower wheel assembly with a mounted pulley and bearing on a shared shaft. In the original design, the bearing and pulley were mounted using traditional press fits. The assembly was located inside the unit casement.

The operating environment for a PTU was demanding: wide temperature swings between off and operating states, humidity, and limited access to lubricant once assembled. Press fits that were correctly sized at installation could relax under these conditions, causing premature failure.

The high assembly forces required by the original press fits created two problems.

First, the bearing had to be located inside the casement—the only position where the press force could be properly applied and the fit secured during manufacture. Inside the casement, the bearing was exposed to the environmental conditions (temperature cycling, moisture, lubricant starvation) most likely to cause premature failure.

Second, once the bearing failed, field repair required removing and disassembling the unit, dealing with the high disassembly forces of the press-fit components, and reassembling with the same high installation forces. This excessive handling meant repair was time-consuming and technically demanding in the field.

Cutaway diagram of a PTU blower wheel shaft

The Solution

How Tolerance Rings HOLD THROUGH EXTREME TEMPERATURE

Two light-duty tolerance rings replaced the press fits for both the pulley and the bearing on the blower wheel shaft. The substantially lower assembly forces required by tolerance rings, compared to press fits, made hand assembly practical.
This changed the architecture of the unit. With hand-assembly retention, the bearing was repositioned outside the casement. Outside the casement, the bearing was no longer exposed to the internal temperature cycling and moisture environment. The bearing failures caused by those environmental conditions were eliminated.
The lower assembly forces also meant the unit could be disassembled and reassembled by a service technician in the field without a press.

Results

What the Data Showed

  • Bearing was repositioned outside the casement, removing it from the primary failure environment
  • Premature bearing failures from temperature fluctuation, moisture, and lubricant starvation were eliminated
  • Hand assembly of both pulley and bearing was achieved, no press required
  • Units became field-repairable, reducing repair time and costs

Engineering Explanation

WHY THE TOLERANCE RING SUCCEEDED WHERE THE PRESS FIT FAILED

Tolerance rings provided retention through spring compression rather than a fixed dimensional interference. Because the clamping force was generated by the ring’s corrugated waves rather than a tight bore-to-shaft fit, assembly and disassembly forces were substantially lower than a press fit sized for the same retention load. This made hand assembly practical and field disassembly reversible.

Customer PTU manufacturer
Application Bearing + pulley on blower wheel shaft
Ring Type BN style (waves outward, securing inner race and pulley to shaft)
Component Replaced Traditional press fits for bearing and pulley
Housing / Equipment Type Packaged thermal unit (PTU), self-contained HVAC
Industry HVAC / packaged thermal equipment

 

ENGINEERING EXPLANATION

Industries