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A bellows can be thought of as an elastic pressure envelope, which works either as a spring or as a fluid containment, often acting as both.
The relationship between load and movement can be constant, within elastic limits, but a bellows is ineffective as a tension spring due to the limited range of elastic movement. It is therefore usual for the ‘stroke’ of the bellows to be in compression only. The spring rate of the bellows must be taken into account in most bellows applications.
They are commonly used in the following devices:
Pressure/ temperature sensing or controlling devices
Temperature
Bellows can be used to control the temperature of every-day products such as refrigerators and ovens. The bellows is connected via a flexible capillary tube to a thermal bulb located on the product. The entire system is filled with a suitable thermo-sensitive fluid for the temperature range.
Temperature changes are sensed by the bulb and transmitted through the capillary tube to the bellows, which contracts or expands to actuate a switch or flow device which governs heat flow to the device, thereby maintaining a set point temperature; a single element control loop.
For different environments the necessary volume change in the sensing fluid is obtained from the expansion of a gas, the vapour pressure of a liquid, or the expansion of a non-volatile liquid.
For high temperatures above 350 deg C an inert gas such as argon may be used, for example to control pilot flames in boilers.
The vapour pressure principle is widely used for above and around room temperature devices. When a liquid is heated the vapour pressure generated in a confined space is readily calculated, and the appropriate combination of liquid and bellows can be specified based on the operating range, sensitivity and pressure criteria.
A partially-filled system can provide more flexibility to obtain the benefits of vapour pressure and liquid expansion, with suitably volatile filling fluids.
Changes in barometric pressure, and friction due to bellows movement may also need to be taken into account.
Pressure
Bellows can be used equally for pressure indication and control, with pressure range usually accomplished at the expense of sensitivity.
Direct pressure design is analogous to the direct thermal control systems above. For example in an oil burner valve system, the fuel oil pressure is applied externally to a sealed bellows unit to directly operate a control valve. Similarly a sealed bellows unit may actuate a steam strap.
Applications requiring barometric compensation are simply achieved by assembling in series the functional bellows with an evacuated bellows of the same size eg. in aerospace applications where altitude affects system performance.
Flexible couplings & actuators
Power Transmission
As a flexible coupling, a bellows is 100% mechanically efficient, , d may be used for example in precision actuator drives with electric motors and mechanical gearing.
The hydroflex coupling, which maybe used to accommodate shaft misalignment in both hand and miniature drives, has properties unequalled by any other coupling. Not only does it take up both angular and lateral misalignment, separately or together, but it does so with complete freedom from backlash.
Compact in size, it provides a constant velocity drive, it has no moving parts and requires no lubricating, servicing or attention.
These characteristics derive from the use of a hydroflex.co.uk bellows, which, while it can be compressed actually and also have its axis displaced, has a high degree of torsional rigidity.
The coupling is designed to allow for the inevitable residual shaft misalignment which occurs in an assembly through normal manufacturing tolerances. It makes expensive and skilled fitting unnecessary and will solve many problems in situations where a solid type coupling can not be used.
A feature of the coupling, of particular interest to certain customers, is its additional use as a "shear pin". By careful selection the coupling can safeguard equipment from extensive damage in the event of a mechanical fault. For example, if used to couple a motor to a gearbox, the coupling would fail if the latter seized, thus preventing the motor from stalling.
Actuator
When used as a flexible pressure envelope, for example as part of a system to transmit fluid motion, one bellows unit can be attached to one end of a tube with another attached at the other end. The size of each bellows can be calculated to provide ratio control without recourse to any other devices.
Similarly one, two or more bellows can be used in a cup assembly for the operation of the next component in series (a valve or switch for example) without any complex ancillary devices to provide amplification of load or movement.
Flexible seals for moving shafts and spindles
A bellows may be use as a shaft seal where axial or flexural movement is required, for example to seal valve spindles or mechanisms where thermal expansion must be accommodated. The bellows may be simply to exclude moisture or dust (for example in transformers and bus bars) or to contain fluids under pressure or vacuum conditions.
Operational Life
The effective operating life of any bellows depends upon a combination of the pressure applied and working deflection. Longevity is achieved by operating the units well within their maximum permissible limits for both deflection and pressure. Hydroflex can offer full facilities for the testing of bellows for life cycle purposes.
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