Heel-Toe (Prying)
From Aero Stress Wiki
Heel-toe (otherwise known as "prying") is an effect of offsetting the tension load on a fastener by some given length. This feature is commonly found in tension clips (small bracket designs) and tension joints.
Contents
Summary
\(\sum{}F=0=P+R-Q\)
\(\sum{}M=0=Rb-Pa\)
\(R=\dfrac{Pa}{b}\)
\(Q=\left(1+\dfrac{a}{b}\right)P\)
Variables
\(P\) - Applied tension load
\(Q\) - Bolt tension load
\(R\) - Toe reaction load (applied at center of bearing pressure)
\(b\) - Toe distance
\(a\) - Heel distance
Strength Correction Factors
- None
Additional Safety Factors
- None
Important Considerations and Limitations
- The bearing pressure distribution is assumed to be linear. Starting at the fastener head (or washer) and extending out to the edge.[1]
- Tension clips should only be used as a last resort. Designs should use shear clips as much as possible.[1]
- Use with light tension loads only.[1][2]
- This arrangement has poor fatigue life.[2]
- Reduce the heel distance (a) as much as possible to reduce bending moments.[1][2]
- Do not use in repeated loading situations. If the tension clip does undergo fatigue cycling a minimum yield (limit) margin of safety of 200% should be maintained.[2]
- If more strength is needed, use a machined fitting (bathtub type).[1][2]
- Only tension bolts should be used with tension clips. Rivets should be avoided.[1]
- A minimum margin of safety of 20% should be maintained for tension clips.[1]
