Assembling one part to another usually requires a third material—screws, rivets, adhesive or filler metal—but it doesn’t have to. In fact, sometimes all you have to do is bend, fold or deform a feature on one part to capture the other.
As long as one of the parts is made of a malleable material—typically metal—engineers can use a press to crimp, stake, swage or clinch it to retain the other part. For added strength, the second part can include a ridge, groove or other feature to retain the material that flows from the first part.
Crimping involves pinching or compressing material around another part. This technique is used to attach terminals to battery cables and fittings to automotive hoses. Crimping is used to assemble solenoids, door locks, sprinkler heads and catheters.
In staking, downward pressure is applied to the end of a post or shaft, creating a head, or to the inside wall of a bore, forming one or more retaining points. The cover of a window lifter motor assembly is staked, as are the guide rings on timing belt pulleys. Bearings are often staked to shafts or bores to supplement the interference fit, and the ends of bolts are sometimes staked to prevent them from loosening.
In swaging and flaring, material from a post or cylinder is moved outward or inward radially to capture an edge or rim. Swaging is used to attach brass pins to circuit boards. It’s also used on seat belt retractors, antilock brake system cartridges and swivel joints. It’s even used on stents for cardiovascular procedures.
In clinching, a punch and die are used to plastically form a mechanical interlock between metal sheets. This technique is used on air bag housings, disk brakes, computer chassis and washing machine cabinets.
Much at Stake
When designing a staked assembly, engineers should start with the strength requirements for the joint, just as they would when designing a bonded or bolted joint. By knowing how strong the joint has to be, engineers can determine how many points must be staked and how much material must be moved. That information, in turn, influences the shape of the tooling and how much force is required to stake the assembly.
The tooling for staking is made of heat-treated steel, which can be coated or uncoated. The shape of the tool is simple. For example, the tool for staking a pinion shaft is shaped like a chisel. To stake a bearing inside a bore, the tool will be slightly less than the diameter of the bore. Teeth extending radially from the tool are designed to push material down from the sides of the bore to hold the bearing in place.
For example, Promess Inc. designed and built a system to stake two bearing cups on a universal joint. The unassembled U-joint has a floating spider mechanism that may shift during the process of inserting and staking the cups. A method had to be found to maintain the spider in the center of the U-joint after both bearing cups have been inserted.
Promess attacked the spider from both sides at the same time. By using two electromechanical presses, the cups can be inserted and the spider can be centered before staking takes place. External feedback sensors locate the center of the joint. Then, both presses simultaneously stake the cups in place. By working together at the same time and speed, the presses apply equal force to both sides of the spider mechanism, ensuring it stays centered within the joint.
The key parameters for the process vary. For a staking application employing a manually operated toggle press, just knowing the ram has reached a certain position may be enough. In most cases, however, engineers want to know a certain amount of force has been applied to the part. For critical applications, engineers will want to monitor every aspect of the operation, measuring both force and displacement to ensure the right amount of force was applied in the right spots at the right time.
Engineers should not to be fooled by the apparent simplicity of the staking process. “If you treat it as a primitive process, the quality is not going to be there,” says Glenn Nausley, president of Promess Inc. “You need equipment that allows you to monitor and control the process.”
A classic application for swaging is a ball joint. The ball is inserted into the socket, and a press swages over the edge of the socket to create a lip that holds the ball in place.
Unlike staking, which is performed with a simple up and down movement, swaging often includes a rotational component. The tooling moves around the edge as the press applies downward force.
Press control is critical. “You have to let metal flow at its own rate,” Nausley explains. “If you push it too fast, you can work-harden it, and it will be become brittle and crack.”
In a Clinch
Clinching produces a round, button-type connection between two to four layers of sheet metal. The process can join metal sheets of different thicknesses or materials. The process can even be used on stainless steel.
Clinching will not damage anticorrosive coatings on the metal, and an adhesive or other material can be located between the sheets. The minimum thickness for any one sheet in the assembly is 0.1 millimeter, and the entire stack can be up to 12 millimeters thick.
The tools consist of a punch and a die. There are two types of dies: solid, fixed-cavity dies, and dies with moving components. The punch forces the layers of sheet metal into the die cavity. The pressure exerted by the press forces the punch-side metal to spread outwards within the die-side metal. Joints can range from 1.5 to 26 millimeters in diameter.