Introduction
Snap fits are all around us and for all the right reasons. Snap-fit joints are easy to manufacture, cheap, and join two parts easily without any exposed joints, or bolt mechanics. Plastic fittings often employ snap joints, and you likely encountered these early on through the release buckles on school bags.
The purpose of this article is to provide essential information about snap-fit design. This will enable you to create snap-fit connections for your project.
What Is A Snap-Fit?
Snap-fit is a method used in product design and production for fastening. The approach involves using Snap-fit joints, either hook or head, to connect parts. This results in a fast and effortless assembly and disassembly without the requirement of any extra tools or fasteners.
Snap-fit joints belong to the assembly category of a part and are often considered as an extension of it. The method of snap fitting is a commonly employed technique for joining flexible components. This involves the insertion of one component’s interlocking feature into a cavity or aligning the interlocking features of both components. Successful attachment requires flexibility in the interlocking features, allowing for bending and insertion to occur.
How Do You Make A Snap Fit?
Understanding the distinctive flexible components of each type of snap fit joint makes it easy to create such joints. Utilizing snap fit joints can decrease the number of connections necessary in an assembly, ultimately reducing manufacturing procedures and expenses.
Snap fit joints work by inserting a protruding part into a recessed area. The recessed area locks onto the protruding part, providing a secure fit.
Snap fit joints can either be permanent or detachable depending on the type of undercut and assembly. When designing snap fits that will be reused multiple times, it is essential to carefully consider the materials used. The continuous application of force and use of the snap fit increases the likelihood of failure due to fatigue stress.
Types of Snap Fit Joints
Annular Snap Fit Joints
Annular snap fit joints are commonly used with cylindrical parts. Annular snap joints function using a small circular protrusion, usually shaped like a ridge. This ridge interlocks with a recess that circles another part.
The primary determinant of the force required to attach two components in annular snap joints is the circular ridge. The annular joints can be classified as either permanent or temporary, depending on the lead angle and returning angle.
In annular snap fits, a 45° return angle typically results in a sturdy yet detachable assembly. If the return angle exceeds 45°, the assembly becomes even more robust but disassembly becomes more difficult. A 90° return angle results in a permanent assembly for annular joints.
Torsional Snap Fit Joints
A torsional snap fit joint involves twisting. It features an interlocking beam resembling a cantilever beam. When enough force is applied, the beam can rotate or twist. The torsion snap fit utilizes spring force to load and secure the mating component.
Snap joints that use torsion function similarly to a seesaw. When a component with a groove is inserted, the beam rotates into position. These snap joints are ideal for temporary connections and can be utilized in assemblies where components need to be frequently disassembled.
Cantilever Snap Fit Joints
Cantilever snap fit joints are popular for connecting thermoplastics. They are economical, reliable, and easy to construct. It is composed of a cantilever beam that has a tapered or chamfered head, allowing for easy insertion and flexibility.
Moreover, the cantilever beam has a recess after the tapered part that imparts a hook-like shape to the head. This hook interlocks with the second mating part in cantilever joints.
U-Shaped Snap Fit
U-shaped snap fit joint is another variation of the cantilever snap fit joint. You may have noticed this feature on the back of your TV remote or on toys. Cantilever joints usually involve inserting the cantilever into a hole in the part.
A U-shape can be used to extend the beam length when there is insufficient space. This will decrease strains and enhance flexibility for repeated assemblies. The hook must be on the outer edge of the beam. This is necessary to remove the sliding mechanism of cantilever joints.
Snap Fit Design Calculations
σmax = mc/I
where M is the maximum bending moment
C is the distance from the point of interest to the neutral axis
I is the moment of inertia
Snap-fit design is incomplete without calculating the fatigue stress, maximum stress, deflection force and many other factors that eventually decide the dimensions of the snap fit joint. While there are many snap fit shapes, I will show a sample calculation for the cantilever snap joint and provide the formulas for other prominent snap fits.
There are two ways to design snap fit joints. You can either choose the material first and adjust the dimensions of your snap fit design accordingly or choose the dimensions first and find a material that can fit the calculated dimensions.
Cantilever Snap Joints
A cantilever snap joint is subject to deflection, strain, and bending. Finding these three elements is essential to the design of a cantilever snap fit.
- The maximum bending stress is given by
σmax = mc/I
where M is the maximum bending moment
C is the distance from the point of interest to the neutral axis
I is the moment of inertia
- The maximum strain is given by
ε = M/IE
where E is Young’s modulus of the material
- For a constant cross sectioned beam the deflection is given by
y=0.67 * ει²/h
where l is the length of the beam
h is the thickness at root
- And the deflection force is given by
P =bh²/6 • Es ε/ι
where b is the width at root
Es is the secant modulus
Sample Calculation
Suppose you are designing snap fits with cantilever beams of a constant cross section beam. You have the arm length (l) = 20 mm,
- Width (b) = 10 mm
- Undercut (y) = 3 mm,
- and An angle of inclination of 30°
You’re supposed to calculate the thickness of the arm and the recommended deflection force for an ABS material
y= 0.67 * ει²/h
h= 0.67 * ει²/y
h= 0.670 • 0.25 • 20² / 3
- Use the deflection force equation
P= bh²/6 • Es ε/I
Es= 1350 N/mm²
P= 102 • 23²/6 * 13500 • 0.25/20
P= 13.98 N
- To find the mating force use this equation
W=P • μ+tan(σ)/1-tan(σ)
W=13.98 • 0.72+tan(30)/1-tan(30)
W=42.91 N
Symbols
y = (permissible) deflection (=undercut)
E = (permissible) strain in the outer fiber at the root
l = length of arm
h = thickness at root
b = width at root
c = distance between outer fiber and neutral fiber (center of gravity)
Z = section modulus Z = I c, where I = axial moment of inertia
Es = secant modulus
P = (permissible) deflection force
K = geometric factor
Torsion Snap Joints
- The shear modulus is given by
G=Es/2(1+ν)
where G is the shear modulus
Es is the secant modulus
ν is the Poisson’s ratio
- The deflection force is given by
P • ι= γ GIp /r
where γ is the shear strain
Ip is the polar moment of inertia
W=P +tan(σ)/1-tan(σ)
W=13.98 • 0.72+tan(30)/1-tan(30)
W=42.91 N
Annular Snap Joints
Annular snap fit joints are used to join symmetrical round parts. In most annular snap fit design, at least one of the components is rigid. Having two elliptic or circular parts which are prone to easy deformation can result in smaller force on the material and in turn a larger undercut. The formulas for annular snap fits are given as follows
- Transverse force is given by
P= y • d • Es • X
where y = undercut
d = diameter at the joint
Es = secant modulus
X = geometric factor
- Mating force is given by
W=P • μ+tan(σ)/1-tan(σ)
where μ = coefficient of friction
α= lead angle
Snap Fit Design Calculations
Snap fit designs are still far from complete even after the calculations and choosing a suitable material. There are a few best practices crucial for the manufacturing processes.
Add Fillets
Fillets are tiny features that reflect elements of a great designer. Sharp corners are the biggest reason for stress concentrations in parts. When designing a snap fit, having sharp corners means most of the structural stress is concentrated on a tiny area which leads to failure or breaking of the part. Adding fillets can reduce stress concentration significantly and the most common use of fillets is at the base of the cantilever. Fillet helps reduce stress concentrations by spreading the load over a larger area.
Taper
Tapering along the length of the snap fit can prolong the joint’s life. Tapering is simply reducing the cross-sectional area of the snap fit along its length. Tapered hook is better than a cantilever hook with a uniform cross section because the latter experiences uneven strains. This can result in exceeding allowable strain and eventual failure. Furthermore, tapered design have the advantage of using less material and reducing manufacturing costs for bulk production.
Add Lugs
Almost all high-quality parts have lugs. Lugs are tiny protruding connecter elements that help with aligning two mating parts. They also act as structural support by bearing part of the shear force.
Increase Width Of Hook
Snap fit joints are in their simplest form load-bearing elements. Snap fit design should be able to resist high loading frequencies and one way to achieve this is by increasing the width of the hook in cantilever snap fits.
Fatigue Life
Snap fittings with poor design considerations often fail due to fatigue stress. Fatigue failure occurs due to repeated loading and is common where snap fits are repeatedly disassembled. Choosing materials with higher yielding strength and making geometric considerations can remove the chances of fatigue failure.
Most Common Applications of Snap Fit Design
Snap fit joints are used in cars that weigh tons to simple plastic accessories found in your room. Some of the common uses of snap fit design are mentioned below.
Toys: Snap fits are most commonly used in toys. Toys need to be light, cheap and mass-produced. Having fasteners that can be part of the toys mold is ideal and snap fit joints are used for this reason.
Pens: Pen caps are a classic example of annular fits.
Strap buckles: Buckles used on most bags, tie down straps, and camping accessories also utilize snap fits.
Enclosures: Many electronic enclosures like USB hubs, and ethernet boxes use snap-fit joints.
Lids and covers: Many covers make use of U-shaped snap fittings to easily cover a storage area or cavity in a part.
3D printing designs: When 3D printing components. Most designs are made of plastics and resins. Using bolts and fasteners can compromise the structural integrity of thin designs and snap fits are a cheaper and better alternative.
Advantages and Disadvantages of Snap Fit Design
Pro
Other than the simplicity of making snap fit joints, they
- Provide easy assembly and disassembly without special tools
- Are long-lasting, if designed with correct calculations and tolerances
- Provide a mechanical joint that can be hidden to improve the aesthetic
- They are economical and require no extra material (other than the plastic being used)
Con
- Provide easy assembly and disassembly without special tools
- Are long-lasting, if designed with correct calculations and tolerances
- Provide a mechanical joint that can be hidden to improve the aesthetic
- They are economical and require no extra material (other than the plastic being used)
FAQs
Q: Can metal be snap-fit joints?
A: Snap fit is a common feature to form plastic to plastic, and plastic to metal connections. For metal snap fitting, the connection is usually metal to metal. Meaning both the recess and the protrusion are metal.
Q: What are snap-fit joints used for?
A: Snap fit joints are used to provide secure connection between any two parts. Common uses of snap fit joints unclude pen caps, plastic enclosures, and toys.
Q: What materials make for good snap fit joints?
A: Plastics with high strength and flexibility can be used for snap fit joints. ABS, nylon and PS are common plastics used for making snap joints.