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Why Loose Piece Contacts?

Why Loose Piece Contacts?
06/01/2010
By Martin Furer, Director of Engineering, Schleuniger Solutions AG and Rob Boyd, Crimping Product Manager, Schleuniger Inc. Featured in U.S. Tech and Wire & Cable Technology June, 2010

Loose Piece Crimping Technology

There are two basic types of electrical terminals – reeled terminals and loose piece terminals, commonly known as loose piece contacts (LPC). Both types are crimped onto the ends of stripped wires to form a mechanical and electrical connection, but that is where the similarity ends. There are significant differences regarding how the crimping and quality processes are automated.

Reeled terminals come on a reel and are connected to a carrier strip with each terminal having the same distance, or pitch, between them. This makes reeled terminals easier to automate than LPC because the applicator tooling incorporates a feeding system that feeds each terminal a fixed distance equal to the terminal pitch. They are available in open or closed barrel configurations.
LPC are packaged in single pieces and shipped in bulk. They are also known as “turned contacts” or “screw-machined contacts”, referring to the manufacturing method used to produce them. They are machined from solid rod and are always the “closed barrel” type, meaning that the area where the wire will be inserted forms an unbroken, 360 degree cylinder. LPC come in various shapes and sizes and are primarily used where quality requirements are very high, such as in the military, aerospace and medical markets.

Advantages of Turned Loose Piece Terminals

Because of their broad contact area, LPC have lower electrical resistance than stamped and formed terminals. With comparable wire cross sections and electrical currents, the heat generated in a LPC is substantially less than with other types of terminals. The voltage drop between the conductor and contact is also lower. Because of this, smaller LPC contacts can be used compared to stamped and formed terminals, saving valuable weight and space in many different products.

Roundness Makes A Perfect Fit

Another advantage of LPC is their precision cylindrical geometry. Male and female connectors mate together perfectly, forming a very reliable gas-tight and spark-free connection. This precision fit makes LPC resistant to intermittent connections caused by vibration as well. Standard pull-force tests almost always show that the wire strands break before they are pulled out of the crimped connection. The type of crimp that is typically used to terminate a LPC is known as a 4-indent crimp, where 4 indents are made at 90 degrees to each other. The specifications and quality requirements for such a 4-indent crimp can be found in the SAE standard (aka “Mil-spec”) – which is mainly used in the military and aerospace industry.

Process Automation

LPC are significantly more difficult to automate than reeled terminals. Accurate and reliable processing of LPC requires that they be automatically sorted and oriented into the proper position before being crimped onto the wire. Inserting a stripped wire into a closed barrel LPC requires an additional axis of movement and a guide system to ensure that all of the individual wire strands are safely inside of the contact before crimping.

Both semi- and fully-automatic systems require a number of automated process steps for reliable crimping of LPC:

  • Sorting and orienting
  • Feeding and positioning into the crimp tooling
  • Stripping the wire end and inserting it into the contact which has already been positioned inside of the crimp tooling.
  • Crimping (4-indent crimp), including quality assurance
  • Extracting the completed wire and LPC assembly from the machine without damaging it

Benchtop Machines Combine All Processes

Each step requires a technical solution which differs greatly from those used when crimping reeled terminals. High-precision benchtop machines such as the one shown in figure 3 combine all processes, including quality assurance, into one machine. The machine sorts and feeds the contacts using a combined drum / track system. An integrated stripping unit with trigger sensor strips the wire end. An optional “re-cut function” can be used to trim the wire, ensuring that the strip length and wire end position are exact.

Short cycle times combined with precision mechanics allow for efficient and precise processing – making an automatic Stripper Crimper the preferred choice for demanding customers, especially in the military and aerospace industries. Depending on production requirements, LPC can be processed on fully automatic systems as well.

Sorting

During sorting, LPC are brought into a defined position for further processing. A proven process is to use a rotating drum, which drops the contacts onto a vibrating feed track. The track is specifically designed for the shape of a given contact, so that they can only “fall” into the track in a defined way and are automatically lined up for further processing. A gripper then positions each terminal into the crimping applicator.

Contact type

The contact type determines how linear feeding and positioning are executed. There are 3 basic contact types, which are distinguished based on their center of gravity and shape. Feeding is either done vertically (crimp zone up or down) or horizontally.

Many different types of contacts can be processed. Quick-change tooling systems reduce changeover times to a minimum – for high productivity even when frequently changing applications.

The three most common contact types are:

Type 1: Vertical feeding with crimp zone up

This contact type has a "collar“ (or shoulder) between the connection area and crimp zone. Its center of gravity is below the “collar” in the connection area.

Type 2: Horizontal feeding

Type 2 contacts have no "collar“ and their center of gravity is not clearly defined. Therefore vertical feeding is not possible, therefore, the terminal must be fed horizontally.

Type 3: Vertical feeding with crimp zone down

This contact type also has a "collar“, but its center of gravity requires "upside-down“ feeding, with the crimp zone down.

Feeding and Positioning: Contact

Contacts move along a vibrating feed track towards the positioning unit. The design of the positioning unit depends on the contact type being processed. The positioning unit consists of a combined tilting- / turning-module with gripper. The gripper unit picks up the contact after it is oriented properly and places it inside of the crimping head.

Type 2 contacts are the most complex to feed and orient since the crimp zone can be on the leading or trailing end as they move down the track. The machine must be able to accurately detect the position of the crimp zone and orient it into the correct position for further processing.

Feeding and Positioning: Wire

Once the contact is correctly positioned inside the crimp applicator, it can be crimped onto the stripped wire. The operator inserts the wire into the machine until the wire end touches the trigger sensor. A gripper automatically closes and holds the wire during the entire processing cycle. The semi-automatic Stripper Crimper will guide the wire strands into the contact and ensure that the wire end is at the programmed insertion depth.

Crimping and Quality Assurance

To ensure the quality of every crimp, modern stripper-crimpers have an integrated crimp force monitoring (CFM) system. The CFM measures force vs. time while crimping a contact onto a wire. The operator teaches the CFM some “good” crimps and the reference curve is stored in the CFM memory. During each crimp cycle, the actual force vs. time curve is compared to the stored reference curve.  Any crimps that do not match the reference curve within preset tolerances are flagged as bad and the CFM stops the machine to allow the operator to investigate and correct the problem. The crimping head is very simple to adjust and calibrate, as only the crimping depth needs to be adjusted.

Additional quality assurance methods and test procedures known from “conventional” crimping, such as pull-force testing or visual analysis using cross sectional views known as micrographs, also apply to indent crimps. Quality criteria for gas-tight crimps with LPC’s include correct strip length, insertion depth and pull-force.

Conclusion

LPC are predominantly used in applications where high-quality connections are needed and very limited space is available. Automating the stripping and crimping processes using semi- or fully-automatic equipment reduces cycle times and increases quality, significantly lowering the cost per termination.