What is a Die spring?
In applications with limited space and especially where the overall height is limited, springs made of rectangular wire are often used. These springs are commonly referred to as die springs.
die springs store more energy in a smaller space than comparable round wire springs. Although the stress distribution around the rectangular cross-section is not as uniform as with the round wire cross-section, the energy storage capacity is higher because more material can be built into the allotted space.
Die springs are mainly used in machine tools. However, they are also suitable for many applications in which high static or shock loads are required or when maximum cycle life is important. Rectangular wire is used to reduce the solid height and increase the space efficiency of the design.
Die springs get their name from their use in punch press die sets, where die sets apply pressure to hold the target material in place while the press perforates the substrate. If the die set does not give the punch compliance, the punch could be damaged; if it were to yield too much, the material would not be perforated. Drop springs are occasionally used in transportation and agriculture.
Oil-hardened steel is a common material for die springs. Chromium alloys are also common materials for compression springs, with a chromium layer increasing wear and corrosion resistance.
Chromium-silicon or chromium-vanadium, which improve dimensional accuracy, minimize individual stress points, and can work at higher temperatures, are less common, but also much stronger. These metals allow a pliable layer of colored vinyl to be applied to the spring, which indicates the working load on the spring.
1. Color Coding
The Die springs are color-coded to indicate the workload. Unfortunately, the manufacturers do not agree to a uniform color code, so a reference card from the manufacturer should be used. Depending on the manufacturer, colors indicate a work area from light to extra difficult.
For example, below is a table with some tool spring manufacturers and the color codes they use. Note that Century Spring Corp. further outlines their coding according to the spring material.
2. Wind Direction
The Die springs are wound either clockwise or counterclockwise. Since the load is applied from the side, the wind direction usually does not matter.
3. Die Spring Ends
Die springs have two basic coil end configurations, closed and open.
In the case of closed-end die springs, the pitch is reduced to the point where the wire end rests on the adjacent coil. This makes the last coil of the spring incomplete and ineffective. It can change the compression of the spring slightly.
Open-end die springs do not have pitch reduction at the point of wire termination. Unless grounded, this creates an uneven surface but maybe unimportant in die machinery.
Both closed-end die springs and open-ended die springs can be ground to create a flattened, perpendicular, reliable load-bearing surface. Below are examples of both open and closed-end die springs with grounded ends.
Tips for Selecting and Using Die Springs
- Determine if springs are used for short run times, average cycle long run, rapid cycle or extra stress.
- Use as many springs in the die as space allows, with the least deflection.
- The faster a spring is cycled, the greater the need to work in the ideal operating range.
- Make sure the hole and rod sizes are correct for the spring. A faulty spring guide leads to kinking and possibly to failure of the spring.
- Preventive maintenance on the dies should be performed regularly and the die springs should be replaced at appropriate intervals to avoid downtime.
- Replace all springs of a die at the same time. This ensures that the load is evenly distributed.
- Do not rework die springs by grinding the inside or outside diameter or by cutting off coils. This could lead to premature failure of the spring and possible tool damage.