Disc spring combinations

As already mentioned at the beginning of this section, disc springs can be combined in many different ways to form packs of single discs stacked facing the same way or spring stacks or packs of single discs stacked facing alternate ways.

The following information is only for disc springs without contact faces. It applies analogously to disc springs with contact surfaces. However, it must be noted that the reduction of the disc thickness from t to t' results in a shortening of the spring pack or the spring stack consisting of spring packs.

Constructing the characteristic

Disc spring pack

For spring stacks consisting of i individual discs stacked facing alternate ways, the deflection is multiplied by a factor of i for a constant load. (Fig. B). The length of the unloaded spring stack of single springs is calculated as L_o = i x l_o. If the friction is disregarded, the following are obtained:

• compression: s_ges = s
• Federkraft: F_ges = n x F

Fig A: Spring pack consisting of n individual disc springs (the deflection is enlarged for clarity)

Disc spring stack

For spring stacks consisting of i individual discs stacked facing alternate ways, the deflection is multiplied by a factor of i for a constant load. (Fig. B). The length of the unloaded spring stack of single springs is calculated as L_o = i x l_o.

If the friction is disregarded, the following are obtained:

• s_total = i x s
• F_total = F

Fig B: Spring stack consisting of i individual disc springs

Progressive characteristic

Progressive characteristic load curves can be obtained with stacks consisting of spring packs with different numbers of discs (high friction) or stacks of single discs of varying thickness and overall height (low friction) as shown in Fig. 3.7 (see handbook).

The progression is achieved because the respectively weaker stack - or relatively weaker spring - is cancelled out and thus no longer contributes to the compression of the stack after reaching the flat position or the deflection limiter (see Fig.).

Fig.: Spring stacks with progressive characteristic load curves and stroke limiters to avoid overload:

a) Bell b) Stop

Information on design

The following boundary conditions should be satisfied for the design of a disc spring stack:

• Single springs stacked in alternate ways are used, if the deflection of the single spring is not sufficient.
• Single springs stacked facing the same way are used if high load charges have to be achieved in limited mounting space.
• A large spring diameter enables low overall heights to be achieved.
• Normally no more than 2 … 4 springs should be stacked to form a pack, since with an increasing number of discs the discrepancies between the calculated and measured characteristic increase considerably due to friction (the influence of friction cannot be taken into consideration in the calculation program).

Guides

Spring packs and spring stacks should be guided. This is done with a guide element such as

• a guide rod (internal guidance, Fig. a)
• a guide sleeve (external guidance, Fig. b) or by
• self-centering devices (balls (Fig. c) or by spring-hardened wire segments)

Fig.: Different types of guides for spring stacks

Both internal and external guides should be polished over their entire length and hardened to at least 55 HRC.

In the case of purely static or infrequently alternating loads, an un-hardened guide element can be used. In spring stacks the force can be applied via either the inner or the outer diameter. It must be borne in mind that in the case of force application via the inner diameter higher surface pressures can occur.

Guides

Both internal and external guidance require some clearance, T, between the spring stack and guide element (see Fig.) to allow displacement of the lubricant and to ensure proper guidance.

Table A shows the total clearance, T, for both types of guides, depending on the disc inner diameter Di (in the case of internal guides) or the disc outer diameter De (in the case of external guides).

Fig.: Spring stack guides a) internal, b) external, clearance T between springs and guide element

Di or De (mm)	Total clearance T (mm)
- up to 16,0	0,2
over 16 up to 20	0,3
over 20 up to 26,0	0,4
over 26 up to 31,5	0,5
over 31,5 up to 50,0	0,6
over 50 up to 80,0	0,8
over 80 up to 140,0	1,0
over 140 up to 250,0	1,6
over 250	2,0

Table A: Total clearance between the disc spring and guide element

Self-centring disc spring stacks:

A guide element is not required for self-centering spring stacks. In practice, 3 types of self-centering disc spring stacks are currently used:

Ball-centered disc spring stacks:

The disc springs have flat annular grooves on the inner and outer diameter, into which hardened steel balls are inserted. The disc spring guide is virtually friction-free. Ball-centered stacks are used primarily with larger disc spring dimensions.

Wire-centered disc spring stacks:

A wire-centered disc spring stack is a more cost-effective alternative to the ball-centred stack. Wire segments are substituted for the balls. However, the friction is slightly higher than with ball centering.

Alternative stack guide:

A retaining ring or T-ring may also be used for centering disc spring stacks.