Machine Design

Insert Design Factors

These requirements should be considered in selecting the best insert: shear strength of parent material, operating temperature, load requirement, presence of vibratory loads, ease of installation, and installed cost.

Boss radius and edge distance: The distance from the center of the hole to the nearest edge is usually equal to the insert OD. Brittle-material inserts that produce high installation stresses may require greater edge clearance.

Material thickness: For blind holes and through holes, hole depth equal to the insert's nominal length is generally needed. When inserts are installed in cored, drilled or tapped holes, hole depth is usually greater than the insert nominal length. Drill point depth, length of tap chamfer, space for chips from self-tapping inserts, and clearance for the installation tooling should be considered.

Floor thickness: The dimension from the bottom of a blind hole to the far side of the material must be adequate to withstand the various machining and insert installation forces. Porous materials must be thick enough to prevent leakage if a leakproof assembly is required.

Locking: Assemblies subject to shock and vibration should be protected by inserts with screwlocking features. Locking adhesives may also be used.

Metallic locks work by using a deformed portion of the insert's internal threads to provide a controlled interference fit with the mating screw. Wire-type inserts have a polygonal resilient coil, while solid inserts have internal threads that are deformed by one or more indentations on the outside of the insert. Reusability with metallic locks is good, with customary life beyond 15 cycles. Some versions can be used for service temperatures up to 1,200°F.

Nonmetallic locks generally incorporate a fiber or plastic (usually nylon) pellet, strip, or collar that forces the mating screw into a frictional lock with the internal thread of the insert. The upper temperature limit is 250°F.

Anaerobic adhesive can be applied to insert threads as a liquid. The locking fluids crystallize to a hard plastic in the absence of air and bond the interfaces together. The hardened plastic must be sheared to separate the parts. Upper temperature limit using this method is 400°F. Other thread-locking adhesives may also be used.

Temperature and cold flow: If the parent material expansion rate is greater than that of the insert, some relaxation or clearance can develop. High operating temperatures may reduce the shearing strength of the base material.

Cold flow or yielding of the parent material over a period of time, especially under load, also affects the insert retention force.

TAGS: Fasteners
Hide comments


  • Allowed HTML tags: <em> <strong> <blockquote> <br> <p>

Plain text

  • No HTML tags allowed.
  • Web page addresses and e-mail addresses turn into links automatically.
  • Lines and paragraphs break automatically.