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Five facts about rub­ber-to-met­al ele­ments

you should consider

15.03.2022   |  Andrej Kisselmann

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Rub­ber-to-met­al buffers are among the most com­mon appli­ca­tions for elas­tomers. They are used when­ev­er machin­ery or equip­ment needs to be pro­tect­ed from vibra­tions and shocks. Although they seem sim­ple, their design is com­plex because there are a lot of influ­enc­ing fac­tors to consider.

Here are five facts about rub­ber-to-met­al ele­ments that com­pa­nies should keep in mind.

1. Strict­ly speak­ing, rub­ber-met­al ele­ments have three components

Rub­ber and met­al do not eas­i­ly form a per­ma­nent bond. While it is pos­si­ble to cre­ate a direct bond through process con­trol and com­pat­i­bil­i­ty of the two com­po­nents, this requires con­sid­er­able effort and know-how. Com­pa­nies typ­i­cal­ly use an adhe­sion pro­mot­er that serves as a chem­i­cal bridge and improves the adhe­sion prop­er­ties of both ele­ments. Strict­ly speak­ing, rub­ber-met­al ele­ments there­fore con­sist not of two but of three ele­ments: Elas­tomer, met­al and adhe­sion pro­mot­er.

It is impor­tant that the adhe­sion pro­mot­er also receives atten­tion dur­ing pro­duc­tion. If it is dam­aged or com­pro­mised, there is a risk that the bond between the rub­ber and met­al will be impaired.

For exam­ple, the adhe­sion pro­mot­er can be dam­aged if it is exposed to high tem­per­a­tures for too long. How­ev­er, injec­tion molds are pre-tem­pered to speed up the vul­can­iza­tion process. There­fore, it is impor­tant to reduce the expo­sure and mold­ing times of rub­ber-to-met­al ele­ments. In this way, you avoid dam­age to the adhe­sion promoter.

2. The project require­ments also apply to indi­vid­ual com­po­nents of a rub­ber-met­al element

Rub­ber-met­al ele­ments con­sist of dif­fer­ent groups of mate­ri­als that are chem­i­cal­ly bond­ed togeth­er. Unlike com­pos­ite mate­ri­als, how­ev­er, the mate­ri­als retain their indi­vid­ual char­ac­ter­is­tics after vul­can­iza­tion. This fact should also be tak­en into account in design and mate­r­i­al selec­tion. All com­po­nents must meet the project require­ments, oth­er­wise the com­bined ele­ment is not suit­able for the intend­ed application.

For exam­ple, both the elas­tomer and the met­al must have the right (shore) hard­ness or grade to meet the expect­ed load. The same applies to resis­tances. If the ele­ment is intend­ed for out­door use, for exam­ple, all com­po­nents must be opti­mized for out­door use to the extent pos­si­ble. The met­al, for exam­ple, should have a cor­ro­sion coat­ing suit­able for the intend­ed use, and the elas­tomer should have appro­pri­ate prop­er­ties in its com­pound composition.

3. The geom­e­try of the over­all sys­tem is decisive

If rub­ber-met­al ele­ments are to car­ry large loads, for exam­ple, in addi­tion to the com­po­nent geom­e­try, the weight dis­tri­b­u­tion / cen­ter of grav­i­ty of the sys­tem is also of great impor­tance. This is the case, for exam­ple, with machine feet. In addi­tion to the com­po­nent geom­e­try and hard­ness, which are deci­sive for the stiff­ness of the bear­ing, the instal­la­tion sit­u­a­tion of the bear­ings and the geom­e­try of the machine also play an impor­tant role in terms of sta­t­ic load and dynam­ic per­for­mance. If the cen­ter of grav­i­ty is exact­ly in the mid­dle, all pedestals are equal­ly loaded. How­ev­er, if the machine is designed asym­met­ri­cal­ly, the load is dis­trib­uted uneven­ly. As a result, indi­vid­ual machine feet are exposed to high­er sta­t­ic and dynam­ic loads than oth­ers. If this cir­cum­stance is not tak­en into account in the design and lay­out of the bear­ings, these com­po­nents can fail in use after a short peri­od of oper­a­tion, which can cause fur­ther dam­age to the machine.

There­fore, cer­tain input vari­ables should be con­sid­ered when design­ing rub­ber-to-met­al ele­ments.
The sta­t­ic load dis­tri­b­u­tion, as well as fur­ther para­me­ters (e.g. fre­quen­cies, ampli­tudes, etc.), are impor­tant input vari­ables, because the over­all per­for­mance depends on them, pro­vid­ed that the com­po­nent is designed correctly.

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Cal­cu­la­tion of the sta­t­ic load dis­tri­b­u­tion of a machine

4. Damp­ing and decou­pling have dif­fer­ent requirements

Rub­ber-met­al ele­ments are main­ly used to reduce shocks and vibra­tions. It is impor­tant to dis­tin­guish between damp­ing and decoupling:

This dis­tinc­tion is impor­tant because it direct­ly affects mate­r­i­al selec­tion. Each elas­tomer has dif­fer­ent chem­i­cal and phys­i­cal prop­er­ties and responds dif­fer­ent­ly to ener­gy input. The stor­age mod­u­lus (G‘) describes the ener­gy that is stored after a force is applied to the bear­ing and can be recov­ered after the load is removed from the bear­ing. The loss mod­u­lus (G“), on the oth­er hand, describes the vis­cous frac­tion of a mate­r­i­al and thus the loss frac­tion of ener­gy that is con­vert­ed into heat by inter­nal friction.

Nat­ur­al rub­ber has a high stor­age mod­u­lus and a low loss mod­u­lus, i.e. it exhibits lit­tle ener­getic dis­si­pa­tion. Con­se­quent­ly, nat­ur­al rub­ber is hard­ly suit­able as a mate­r­i­al for damp­ing ele­ments, because it con­verts only lit­tle kinet­ic ener­gy into ther­mal ener­gy. On the oth­er hand, it has excel­lent decou­pling prop­er­ties. With butyl, the reverse is true. Put sim­ply, it has a high loss mod­u­lus and a low stor­age mod­u­lus, so it is very well suit­ed for damp­ing components.

When select­ing the base poly­mer, in addi­tion to input vari­ables such as exci­ta­tion fre­quen­cies, dynam­ic or sta­t­ic loads or ambi­ent tem­per­a­tures, you must there­fore also con­sid­er whether the ele­ment to be designed is to damp or decou­ple later.

5. Rub­ber-met­al ele­ments are dif­fi­cult to recycle

Rub­ber-met­al com­pounds can only be sep­a­rat­ed from each oth­er with a great deal of effort. This makes the recy­cling of such prod­ucts dif­fi­cult. To dis­pose of both mate­ri­als sep­a­rate­ly requires a com­plex recy­cling process with high ener­gy input, which is hard­ly eco­nom­i­cal­ly jus­ti­fi­able. For this rea­son, rub­ber-met­al ele­ments are usu­al­ly land­filled rather than recy­cled. This has an impact on the company’s sus­tain­abil­i­ty goals.

It is there­fore advis­able to ensure the longest pos­si­ble ser­vice life when design­ing rub­ber-to-met­al ele­ments. The mate­ri­als used should be robust and resis­tant to aging. Wear and tear should be min­i­mized (if pos­si­ble) at the geo­met­ric lev­el. This not only reduces waste, but also low­ers main­te­nance and repair costs.

Con­clu­sion

Rub­ber-to-met­al ele­ments are by no means just com­ple­men­tary com­po­nents to pro­tect motors or pumps from shocks and damp­en noise. There are many fac­tors to con­sid­er in their design, from exci­ta­tion fre­quen­cies to sta­t­ic loads to the dis­si­pa­tive prop­er­ties of the base poly­mer. Those who under­es­ti­mate these com­plex­i­ties risk fail­ure dur­ing oper­a­tion, which in the worst case can result in fur­ther dam­age to machinery.

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Author: Andrej Kisselmann

Andrej Kissel­mann is a grad­u­ate of the con­tin­u­ing edu­ca­tion pro­gram in rub­ber tech­nol­o­gy at Leib­niz Uni­ver­si­ty in Hanover. Since 2011, he has been work­ing for Jäger in cen­tral prod­uct man­age­ment as well as in tech­ni­cal sales.

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