Table of Contents
Introduction
Delrin bushings are widely used across industries for their combination of low friction, high wear resistance, and excellent mechanical properties. Selecting the right size and diameter is critical—too tight and the bushing can bind or generate excess heat; too loose and it will wear prematurely or cause misalignment. This guide provides a thorough method for sizing Delrin bushings, covering material properties, load and speed considerations, measurement techniques, installation best practices, and comparisons with alternative bushing materials. By following the steps outlined here, engineers and maintenance professionals can ensure reliable, long-lasting performance in any rotating or sliding application.
Understanding Delrin Bushings
Delrin is the brand name for polyoxymethylene (POM) homopolymer, a high-performance acetal plastic. Unlike acetal copolymer, Delrin offers superior stiffness, tensile strength, and fatigue resistance, making it ideal for precision bushings. A bushing is a cylindrical sleeve that supports a rotating or sliding shaft, reducing friction and wear between moving parts. Delrin bushings provide a self-lubricating surface, low moisture absorption, and dimensional stability across a wide temperature range. They are commonly used in automotive components, conveyor systems, office equipment, and agricultural machinery.
Key Properties of Delrin for Bushing Applications
Low Friction and Self-Lubrication
Delrin has a coefficient of friction comparable to nylon and PTFE, typically 0.2–0.3 against steel under dry conditions. Its natural lubricity eliminates the need for external grease in many applications, reducing maintenance and contamination risks.
High Wear Resistance and Mechanical Strength
With a tensile strength of about 10,000 psi and excellent creep resistance, Delrin bushings can support heavy loads without deforming over time. The material withstands repeated stress cycles, making it suitable for continuous-duty machinery.
Dimensional Stability
Delrin absorbs less than 0.2% moisture, ensuring minimal swelling or shrinkage in humid environments. This stability allows tight tolerances to be maintained throughout the bushing’s service life.
Thermal and Chemical Resistance
Delrin operates continuously from -40°F to 180°F and can withstand short-term exposure up to 250°F. It resists many solvents, fuels, and weak acids, but should not be used with strong acids or long-term exposure to moisture above 140°F.
Factors to Consider When Choosing Size and Diameter
Application Load and Pressure
The total radial or axial load must be distributed across the bearing surface area. The pressure (load divided by projected area) should stay below the material’s PV limit. For Delrin, a common PV limit is around 3,000 psi·ft/min for continuous operation. Higher loads require larger diameters or longer bushings to spread the force.
Operating Speed and PV Rating
Speed influences frictional heat generation. The product of pressure (P) and velocity (V) – the PV factor – must stay within the material rating. At higher speeds, even moderate loads can exceed the PV limit. Use the formula: PV = (load in lb ÷ (ID × length in inches)) × (shaft rpm × π × ID / 12). Ensure the result is below the material’s maximum PV.
Environmental Conditions
Temperature fluctuations may cause differential expansion. Delrin expands about 4.5×10⁻⁵ in/in/°F. In applications with wide temperature swings, allow additional clearance of 0.001–0.003 inches per inch of shaft diameter. Chemical exposure can degrade the material – consult chemical compatibility charts before selecting Delrin for aggressive environments.
Fit and Tolerances
The bushing must fit securely in its housing (interference fit) and have controlled clearance on the shaft. A typical press fit for the outer diameter is 0.001–0.003 inches interference for diameters up to 1 inch. The inner diameter clearance should be 0.001–0.005 inches for general applications; tighter clearances may be used for low-speed, high-precision systems. Always measure actual shaft and housing diameters with precision calipers or micrometers.
How to Measure and Select the Right Size
Follow these steps to ensure correct sizing for your Delrin bushing.
Step 1: Measure the Shaft Diameter
Use calipers to measure the shaft at three points along its length and average the reading. Record to the nearest 0.001 inch. If the shaft has wear, measure the undamaged section or replace the shaft.
Step 2: Measure the Housing Bore
Measure the inside diameter of the hole where the bushing will be installed. Clean the bore thoroughly before measuring. For blind holes, use bore gauges.
Step 3: Determine Inner Diameter (ID) and Clearance
The bushing ID should match the shaft diameter plus clearance. For shaft diameters less than 1 inch, add 0.001 to 0.003 inches; for larger shafts, add 0.002 to 0.005 inches. Example: a 0.5000 inch shaft would use a bushing with ID 0.5005–0.5030 inches.
Step 4: Select Outer Diameter (OD) and Interference
The bushing OD should be 0.001–0.003 inches larger than the housing bore (for metal housings). For plastic housings, use light interference or slip fit. The wall thickness should be at least 1/8 inch for bushings under 1 inch ID, scaling up proportionally.
Step 5: Choose Length
Bushing length is typically 0.5 to 2 times the shaft diameter. Longer bushings handle higher loads and reduce bearing pressure. Ensure the bushing does not interfere with adjacent components.
Standard Sizes of Delrin Bushings
Many suppliers offer standard sizes, which simplify procurement and reduce cost. The table below lists common dimensions (inches). Always verify availability with your supplier.
Common Delrin Bushing Dimensions
- Inner Diameter (ID): 1/8, 3/16, 1/4, 5/16, 3/8, 1/2, 5/8, 3/4, 1
- Outer Diameter (OD): 1/4, 3/8, 1/2, 5/8, 3/4, 1, 1-1/4, 1-1/2
- Length: 1/4, 3/8, 1/2, 5/8, 3/4, 1, 1-1/2, 2
Stock sizes typically have wall thicknesses of 1/8 or 3/16 inches. For non-standard dimensions, custom machining is available. For a comprehensive selection, refer to suppliers like Misumi or McMaster-Carr.
Custom vs. Standard Sizes
Standard bushings are cost-effective and readily available, but custom sizes allow optimization. When the load or space constraints require non-standard dimensions, Delrin can be turned, bored, and cut to length with standard lathe operations. Custom bushing orders typically have a minimum quantity of 10–50 pieces. Work with an experienced machine shop to achieve tolerances of ±0.001 inch. For high-volume production, compression molding may be used, though machining is preferred for tight tolerances.
Installation Best Practices
Press Fit Installation
For interference fits, use a press or hydraulic arbor press to avoid damaging the bushing. Apply force evenly to the bushing face, not the edge. Chamfer the lead-in of the housing bore and the bushing OD to prevent galling.
Clearance and Lubrication
Even though Delrin is self-lubricating, adding a light film of grease at assembly reduces initial run-in wear. For high-speed or edge-load applications, consider using a lubricant compatible with acetal (e.g., PTFE grease).
Thermal Considerations
During assembly, cooling the bushing or warming the housing can ease press fitting. Delrin’s higher thermal expansion coefficient means this technique is less effective than for metals; typically, press fits are done at room temperature.
Inspection
After installation, check that the bushing is not deformed and that the shaft rotates freely. Measure the ID after pressing to ensure it hasn’t collapsed. A go/no-go gauge can confirm proper clearance.
Comparing Delrin to Other Bushing Materials
Choosing the right bushing material depends on application requirements. The following points highlight trade-offs.
| Material | Advantages | Disadvantages |
|---|---|---|
| Delrin (Acetal Homopolymer) | Low friction, high strength, low moisture absorption, good fatigue life | Limited temperature range (under 185°F continuous), poor resistance to strong acids |
| Nylon 6/6 | Higher toughness, better impact resistance, lower cost | Absorbs moisture (swells up to 1%), lower dimensional stability |
| Oil-Impregnated Bronze | Very high load capacity, excellent thermal conductivity | Heavier, requires oil wicking, can be noisy, higher friction without oil |
| PTFE (Teflon) | Lowest friction, wide temperature range, chemical inertness | Low load capacity, high wear rate, softness leads to creep |
For most general-purpose applications, Delrin offers the best balance of strength, wear, and cost. For wet or corrosive environments, PTFE or filled nylon may be preferred. For extreme loads, bronze bushings are traditional but require regular lubrication.
Common Applications of Delrin Bushings
- Automotive: Door hinges, seat mechanisms, pedal assemblies, throttle linkages – benefit from low friction and no grease.
- Conveyor Systems: Roller bushings, chain guides – run quietly and resist dust.
- Office Equipment: Printer rollers, copier guides – precise motion without maintenance.
- Agricultural Machinery: Implement linkages, pivot points – withstand dirt and intermittent loads.
- Medical Devices: Adjustable bed mechanisms, linear slides – require smooth motion and material approval.
Conclusion
Selecting the right size and diameter of Delrin bushings involves understanding load, speed, environment, and fit requirements. By measuring accurately, calculating PV factor, and choosing appropriate clearances, you can ensure optimal performance and longevity. Standard sizes simplify procurement, but custom dimensions are feasible for specialized designs. Delrin’s inherent properties—low friction, high strength, and moisture resistance—make it a top choice for countless applications. When in doubt, consult technical resources from manufacturers such as DuPont or Engineering ToolBox for detailed PV charts and chemical compatibility. Properly selected and installed Delrin bushings will deliver years of reliable service with minimal maintenance.