What is a Press-Fit Application?
A press-fit application is a joining application in which two components are forced together. Typically, this involves a part that’s minimally larger than a hole or fitting in another component and pressing the two together with enough force to create an interference fit between the two pieces. The mating piece could be cylindrical, which is often referred to as a pin or shaft, but it does not have to be – parts of different shapes can be mated together with the importance being that there is slight overlap or negative clearance between the pieces. With press-fit or interference fit connections, the size of the component and the fitting hole are altered for the pieces to fit together, and both sides will exert force on the other in attempt to revert back to their original shape. In this way, small interference creates a great amount of pressure and friction between the parts, resulting in a strong, often permanent connection.
The quick, clean, heat-free and consistent press-fit application results in many advantages over methods like soldering, brazing or welding that create similar holds:
- More flexibility with design – not having to account for spatial design allowances for someone to properly access an area for welding, brazing or soldering pieces together creates better flexibility with the product design.
- Lower overall production costs – Press machinery can be costly upfront, depending on the requirements for an operation, but the cost of machinery, setup and maintenance are typically much lower than the overall cost of materials, equipment and the cost of manual labor for manual welding, soldering and brazing operations. Using machinery for press fit applications also requires far less training for workers, which is an added incentive to minimize overall labor costs.
- Less risk of flaw or error – an imperfect application of filler material for soldering, brazing or welding can create dry-joints, cracks or voids in a connection. Eliminating the need for filler and also human error in the joining process drastically reduces the chance of error.
- Better, more consistent connection – With proper alignment and force regulated with each application, press-fitting parts creates tight, consistent connections each time. Force and distance monitoring can be added to press equipment to further ensure consistency and reliability.
- Suitable for automation – Often, one or more components will need to be assembled together before it can be moved along into the next step in an assembly process. With the proper tooling, monitoring and controls for your factory equipment, press-fit applications can be worked into an automated single application process or as part of a fully automated turnkey automation system, further increasing efficiency and profitability with production.
- More worker friendly and better for the environment – Filler compounds used in soldering, brazing and welding processes can contain lead or cadmium, and heated together with the parts and pieces which may have been cleaned with chemicals or coated, various fumes and gasses are released into the air with each manual application. Exposure to these fumes can be toxic to workers and also contaminate the surrounding air. Heating applications also produce sparks, which can cause fires or explosions, creating a risk of bodily harm to workers and possible damage to surrounding equipment. Press-fit applications remove the need for fillers, heat or flames, and also do not disturb material coatings, greatly reducing risk to operators and the environment.
Because of its clear advantages in creating a clean, quick, and tight connection with consistent reliability, press-fit applications are replacing previous methods of connection across a wide range of industries. You can find press-fit assembly operations in automotive, medical device, HVAC, appliance, lighting and computer manufacturing, and even in other non-manufacturing operations like piping system repair where the fast, flame-free method limits leaking and potential explosion hazards. Press-fit technology has seen quickly increasing demand in the manufacturing and assembly of electrical components across various industries, as the press-fit connection has been found to support higher currents of electrical energy more consistently than soldering or other pin fitting methods.
Key Considerations for Press-Fit Applications
When attempting to join materials with press-fit technology in your manufacturing process, there are some key elements to consider for a successful operation. Everything from the calculated force, to the environment of the press application can affect the results of your interference fit, and your press-fit application must be carefully planned and executed for accurate and reliable results.
When considering the use of press-fit application in your manufacturing or assembly production, the first thing to take into mind is the materials you are using. Press-fit is not an all-inclusive application. A range of materials can be press-fitted together and dissimilar materials can be combined, but understanding the pressure requirements is key to selecting the right materials to work with that will create a stable hold. When fitting elements of different materials together, it is important to consider how temperature will affect each of the mating pieces. If one element will shrink or expand faster in colder or warmer environments, accurate pressure will be difficult to maintain and the fit may not be reliable, even if you initially experience a good press-fit at the time of insertion. Certain plastics should not be used in press-fit applications altogether if they have a relatively high flow rate and will distort under constant pressure, since maintaining pressure is the key to a long-lasting interference fit.
For the pieces to fit together and create the desired hold, having the exact diameter ratio, pressure and force requirements calculated for the process is essential. The design element that is most relevant in press-fit operations is tolerance, or the amount of overlap between your shaft diameter and its fitting. Generally larger elements will have a larger range of allowable tolerance whereas smaller elements will have littler to near zero range of tolerance. The tolerance level also depends on how critical the connection will be to the function of the part. Less critical parts will not require as tight of tolerance or hold between the pieces. Pieces designed incorrectly will not produce a suitable interference fit, or in worse cases, will not fit together no matter how much force is applied, which can lead to machine damage and/or create risk of damage to operators and surroundings.
Even the best engineers cannot forgo testing their designs and calculations in a real world setting. Everything from a cleaner used on the material, a particular coating, inconsistencies in your part materials or even the temperature or humidity of the work environment can affect the resulting fit and pressure within a press-fit application. Therefore it is essential to test your press-fit application in its final setting with the machinery, materials and environmental factors that will be used in the application day-to-day to determine actual results.
The Right Press Machinery for Your Press-Fit Application
Selecting machinery with the correct force and tooling is important to maintaining a consistent operation. Many press machines can perform press-fit applications including air presses, hydropneumatic presses or hydraulic presses. Higher force will be required for solid parts, opposed to a hollow shaft, and the force will have to be exactly right for softer materials like plastics that need enough force to create the interference fit without causing the material or part to become disfigured. The right tooling is also essential to ensuring accuracy of alignment and fit with each press-fit application. Force and distance monitoring elements can be applied to your press machinery to help maintain quality and ensure repeatability occurs with each insertion. This often leads to increasing reliability, efficiency, reduced material waste and ultimately helps maintain higher product standards.