GB2145879A

GB2145879A - Solenoid actuator with loose-fit armature

Info

Publication number: GB2145879A
Application number: GB08323453A
Authority: GB
Inventors: Michael Clift; Gerald Brown
Original assignee: Individual
Current assignee: Individual
Priority date: 1983-09-01
Filing date: 1983-09-01
Publication date: 1985-04-03
Also published as: GB8323453D0

Abstract

A solenoid has a coil (11) operating on an armature (12) and shaft (15), moving in co-operating bores in a mild steel housing. In order to reduce or prevent hum or chatter of the armature and shaft, their clearances in the bores are increased, e.g. to provide 0.48 mm clearance on each side of the shaft 15 and 0.45 mm on each side of the armature 12. Thus a degree of instability is built into the armature and shaft such that they tend to fall into a tilted position contacting the bores at either end. This bridges the air gap forming a complete magnetic path holding the armature and shaft stationary. A portion 32 of the armature has a closer tolerance in the bore. The actuator may be part of a valve. <IMAGE>

Description

SPECIFICATION Solenoid This invention relates to solenoids and to a method of reducing chatter in items operated by solenoids.

The invention provides a solenoid comprising a coil and an elongated armature movable by the solenoid in a bore, the armature being a loose fit in the bore whereby when the solenoid is energized it falls into a tilted position in the bore bridging the air gap at each end of the bore.

Preferably the armature includes a shaft for operating a valve, which shaft also moves in the bore and when in the tilted position directly contacts one side of one end of the bore. The shaft and bore may be of mild steel.

The armature may be copper plated over an area which may contact the other end of the bore, when in the tilted position.

The loose fit may comprise clearances of between 0.1 mm and 0.9 mm on each side of the armature and shaft.

A specific embodiment of the invention is shown in the accompanying drawings, in which: Figure 1 is a side section through a solenoid-operated valve, and Figure 2 is a circuit diagram showing the power supply to the solenoid of Fig. 1.

Referring first to Fig. 1, a solenoid has a coil (11) operating on an armature (12). The armature is generally cylindrical and is movable against the resistance of a spring (13).

Mounted on one end of the armature is a mild steel shaft (1 5) carrying a valve head (16) cooperating with a valve seat (17) (shown only partially). The position of the armature, shaft and valve head when the valve is closed is shown in full lines, and when it is open in dotted lines.

The Armature and shaft move in co-operating bores in a mild steel housing (18) attached to one end of a housing (19) for the coil.

It has in the past been found that solenoid arrangements of this type tend to hum or chatter, due to rapid rattling movements of the armature and shaft in their bores as the applied voltage (and consequently the magnetic attraction) varies in its cycle. To reduce this hum the applied voltage as shown in Fig.

2 is subject to half wave rectification by a diode (20) with a further diode (21) connected across coil (11) which allows the current from the collapsing field to continue during the OFF half cycle of the rectified current. This has the effect of maintaining the magnetic field during the OFF half cycle and thus reducing the tendency to hum. However, with small solenoids the inductance of the coil is too low to prevent the current from reducing to a value such that the spring force exceeds the electromagnetic force during the OFF half cycle and when this occurs the chatter or hum still happens.

Previous efforts to prevent this hum have concentrated on reducing clerances so that the available movement is less, but these efforts have not been successful because sufficient clearance has to be left for easy movement of the armature and shaft in their bores.

According to the invention, the opposite route is taken and the clearances in the bores are opened up sufficiently to allow the armature and shaft, when energized and pulled back into the open positions, to take up the tilted position shown in the dotted lines. In this position the forward edge of the bore at (30) contacts the surface of the shaft and the rearward edge of the armature at (31) contacts the surface of the bore. It will be noted that in this position there is a mild steel to mild steel contact at (30). The armature at (31) is plated with copper. Thus the air gap is bridged at both ends and a complete magnetic path is established in magnetic metal.

This tends to act like a "keeper" for the magnetic circuit. The small remaining magnetic forces can keep the armature and shaft stable in the tilted position in direct contact, where they could not keep the armature stable floating across air gaps.

Because the clearances are large, the armature and shaft are not closely located radially and must fall to one side or another when the armature is energized. They will then be held stably in that position by the magnetic field.

In the embodiment shown a clearance of 0.48 mm on each side of the shaft is allowed and a clerance of 0.45 mm on each side of the armature is allowed, except at the end (32) opposite to the shaft where a clearance of 0.02 mm locates the armature at that end only.

1. A solenoid comprising a coil and an elongated armature movable by the solenoid in a bore, the armature being a loose fit in the bore whereby when the solenoid is energised it falls into a tilted position in the bore closing the air gap at each end of the bore.

2. A solenoid as claimed in claim 1, wherein said bore is formed in a housing of material capable of-maintaining a magnetic circuit.

3. A solenoid as claimed in claim 1 or claim 2, wherein said loose fit is sufficient to make the armature radially unstable whereby it must fall to one side or the other when energised by the solenoid, thus contacting the bore.

4. A solenoid as claimed in claim 3, wherein said loose fit provides a clearance of between 0.1 mm and 0.9 mm on each side of the armature.

5. A solenoid as claimed in any of claims

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Solenoid This invention relates to solenoids and to a method of reducing chatter in items operated by solenoids. The invention provides a solenoid comprising a coil and an elongated armature movable by the solenoid in a bore, the armature being a loose fit in the bore whereby when the solenoid is energized it falls into a tilted position in the bore bridging the air gap at each end of the bore. Preferably the armature includes a shaft for operating a valve, which shaft also moves in the bore and when in the tilted position directly contacts one side of one end of the bore. The shaft and bore may be of mild steel. The armature may be copper plated over an area which may contact the other end of the bore, when in the tilted position. The loose fit may comprise clearances of between 0.1 mm and 0.9 mm on each side of the armature and shaft. A specific embodiment of the invention is shown in the accompanying drawings, in which: Figure 1 is a side section through a solenoid-operated valve, and Figure 2 is a circuit diagram showing the power supply to the solenoid of Fig. 1. Referring first to Fig. 1, a solenoid has a coil (11) operating on an armature (12). The armature is generally cylindrical and is movable against the resistance of a spring (13). Mounted on one end of the armature is a mild steel shaft (1 5) carrying a valve head (16) cooperating with a valve seat (17) (shown only partially). The position of the armature, shaft and valve head when the valve is closed is shown in full lines, and when it is open in dotted lines. The Armature and shaft move in co-operating bores in a mild steel housing (18) attached to one end of a housing (19) for the coil. It has in the past been found that solenoid arrangements of this type tend to hum or chatter, due to rapid rattling movements of the armature and shaft in their bores as the applied voltage (and consequently the magnetic attraction) varies in its cycle. To reduce this hum the applied voltage as shown in Fig. 2 is subject to half wave rectification by a diode (20) with a further diode (21) connected across coil (11) which allows the current from the collapsing field to continue during the OFF half cycle of the rectified current. This has the effect of maintaining the magnetic field during the OFF half cycle and thus reducing the tendency to hum. However, with small solenoids the inductance of the coil is too low to prevent the current from reducing to a value such that the spring force exceeds the electromagnetic force during the OFF half cycle and when this occurs the chatter or hum still happens. Previous efforts to prevent this hum have concentrated on reducing clerances so that the available movement is less, but these efforts have not been successful because sufficient clearance has to be left for easy movement of the armature and shaft in their bores. According to the invention, the opposite route is taken and the clearances in the bores are opened up sufficiently to allow the armature and shaft, when energized and pulled back into the open positions, to take up the tilted position shown in the dotted lines. In this position the forward edge of the bore at (30) contacts the surface of the shaft and the rearward edge of the armature at (31) contacts the surface of the bore. It will be noted that in this position there is a mild steel to mild steel contact at (30). The armature at (31) is plated with copper. Thus the air gap is bridged at both ends and a complete magnetic path is established in magnetic metal. This tends to act like a "keeper" for the magnetic circuit. The small remaining magnetic forces can keep the armature and shaft stable in the tilted position in direct contact, where they could not keep the armature stable floating across air gaps. Because the clearances are large, the armature and shaft are not closely located radially and must fall to one side or another when the armature is energized. They will then be held stably in that position by the magnetic field. In the embodiment shown a clearance of 0.48 mm on each side of the shaft is allowed and a clerance of 0.45 mm on each side of the armature is allowed, except at the end (32) opposite to the shaft where a clearance of 0.02 mm locates the armature at that end only. CLAIMS

5. A solenoid as claimed in any of claims 1 to 4, wherein said armature includes a shaft of reduced diameter which also is a loose fit in another bore whereby when the armature falls into the tilted position, the shaft contacts a side of its bore.

6. A solenoid as claimed in claim 5, wherein the armature hs a clerance of 0.45 mm on each side in its bore over the major part of its length and the shaft has a clearance of 0.48 mm in its bore.

7. A solenoid as claimed in claim 5 or claim 6, wherein the shaft is of mild steel, and the armature is copper plated at the location where it contacts the bore.

8. A solenoid as claimed in any of claims 2 to 7, wherein said housing is of mild steel.

9. A solenoid as claimed in any of claims 1 to 8, adapted to open and close a valve, a movable part of which is secured to move with the armature.

10. A solenoid-operated valve substantially as described hereinbore with reference to the accompanying drawings.