GB2030770A - Timing device - Google Patents

Abstract

A microwave oven timer has a cam 15 and a double gear 26, 27 manually rotatable together by shaft 13 to select a cooking time, and a motor (not shown) for driving shaft 17 and returning the cam and gear back to a zero position at which microswitch 16 is OFF. The return speeds include a faster speed provided by gear 26 which is permanently engaged with friction-driven gear 20, and a slower speed provided by gear 27 which has teeth over only part of its circumference, and which comes into engagement with friction-driven gear 21 only when the cam is rotated to select more than 5 minutes cooking time. In order to enable the engagement of gear 27 to take place smoothly the first few teeth (30, Fig. 3) are rendered radially resilient by a circumferential slot (31) thereunder. A lever 34, interposed between cam 15 and button 35, strikes a bell (33, Fig. 1) when cam 15 opens switch 16. <IMAGE>

Description

SPECIFICATION Timing device This invention relates to a timing device of the kind in which a rotatable cam means performs an operation at a predetermined position thereof and in which the cam may be rotated at one speed during a certain arc of its rotation and at another speed during a succeeding arc of its rotation. Such timing devices will be referred to hereinafter as "timing devices of the kind described".

The invention provides a timing device of the kind described having a gear provided with gear teeth only over an arc of its circumference, said gear being rotatable with said cam and said arc corresponding to one of said arcs of rotation of the cam, the gear teeth moving into mesh with a first drive gear when the cam enters said certain arc and being out of mesh with said first drive gear when the cam is in said succeeding arc, at least the tooth first engaged by said first drive gear as it moves into mesh therewith being springmounted so as to be resiliently movable to allow synchronization with the teeth of the drive gear as the gears engage.

Said spring mounting may be provided by undercutting the said tooth or teeth, the gear being formed of resilient material.

The timing device may comprise a microwave oven timer in which longer cooking times are associated with said one speed and shorter cooking times with said other speed, said one speed being slower than said other speed.

Said gear may be integral with another gear forming a compound gear mounted on the same shaft as the cam and there may be a second drive gear mounted on a common spindle with said first drive gear, said second drive gear being permanently engaged in said other gear of the compound gear, and said first and second drive gears may be frictionally driven by said common spindle whereby one or other drive gear may slip relative to the spindle.

A specific embodiment of the invention is shown in the accompanying drawings, in which: Figure 1 is a view of a microwave oven timer, Figure 2 is a part-sectional view on the line A-A of Fig. 1 to a larger scale, with a motor and gearbox of Fig. 1 removed, Figure 3 is a detail of a compound gear of Fig. land Figure 4 is a side view of the compound gear of Fig. 3.

The timer comprises a motor 11 and gearbox 1 2 connected to rotate a shaft 1 3 through a complex mechanism described in more detail hereinafter. The shaft 1 3 carries a cam 1 5 (Fig. 2) operating a microswitch 1 6 which turns the microwave oven on and off.

To select a cooking time shaft 1 3 is turned via a manual knob (not shown) from a zero position to an angular position related to the chosen cooking time. In this movement the cam 1 5 is turned to a position maintaining the microswitch 'on'. The motor is also energized by the microswitch and commences to turn the cam and shaft to return them to the zero position at which the microswitch is 'off'.

The speed of their rotation is such that at the end of the chosen cooking time the cam reaches its zero or "off' position and the oven and motor are de-energized.

Microwave ovens may be required to operate for relatively long periods (e.g. up to 30 minutes) or for short periods, possibly of 5 minutes or less, and it is desirable that the timer be capable of timing both the longer periods and also the short periods which require greater accuracy of timing than the longer periods. Accordingly, a differential speed is introduced to the drive mechanism so that for chosen cooking times of less than 5 minutes the shaft and cam rotate at a faster speed, while when cooking times over 5 minutes are selected the shaft and cam rotate at a slower speed until 5 minutes cooking time remain and thereafter at the faster speed.The angular movement of the shaft 1 3 required to select the cooking times reflects this speed variation and an index (not shown) against which the manual rotation is measured has a larger scale for the 5 minute cooking times and a closer scale for time exceeding 5 minutes. The shaft can therefore be positioned more accurately against the larger scale for selected cooking times up to 5 minutes.

To achieve the differential speeds, the gear box 1 2 is connected to drive a spindle 1 7 (Fig. 2) which carries two separate collars 18, 19, each collar carrying a gear 20, 21 providing a different gear ratio. The collars are split lengthwise, and formed of a resilient material which allows them to be diametrically compressed in the sense to close the slits. A spring clip is wound around the periphery of each collar to compress it into frictional driven engagement with the spindle, but the frictional engagement may be overcome so that one or other of the collars slides on the spindle, thus allowing the selection of one or other gear ratio.

Shaft 1 3 carries a compound gear 25 secured to rotate with the shaft. Compound gear 25 seen best in Figs. 3 and 4, has two gears 26, 27. Gear 26, of smaller diameter than gear 27, is permanently in mesh with gear 20 to provide the faster of said differential speeds. Gear 27 as seen best in Fig. 3 carries gear teeth around only 158 of its periphery, the remainder of the periphery being flat and of smaller diameter than the roots of the gear teeth. When the gear teeth are located adjacent the gear 21, the gears mesh, but when the flat part is located adjacent gear 21 there is no meshing. The meshing of gears 21 and 27 provides the slower of said differential speeds.

When the shaft 1 3 is manually rotated by 158 from its zero position, a position select ing 5 minutes cooking time, then the gear 27 is in a position in which the flat part is adjacent gear 21 and its first gear tooth 30 is close to but not meshing with gear 21. In this condition a faster drive is supplied to the shaft 13 via gears 20, 26 and gear 21 rotates out of mesh with gear 27. The return motion of shaft 1 3 to the zero position is accomplished by gear 20 driving gear 26, gear 21 rotating idly, the return taking 5 minutes at the faster speed.

When the shaft 1 3 is manually rotated through more than 158 from its zero position to a position selecting more than 5 minutes cooking time, then gear 27 is turned to a position in which its gears mesh with gear 21.

The return motion of the shaft 1 3 is therefore accomplished at the slower speed. During the slower speed, the gear 20 is forced to slip on the spindle 17, the frictional engagement being designed to be overcome by the superior torque from gear 27. When 5 minutes cooking time remain the gear 27 has rotated to the position in which its first tooth 30 disengages from gear 21 and the faster speed becomes operative for the last 5 minutes.

The engagement of tooth 30 with gear 21 is a point of difficulty when the shaft 1 3 is being manually rotated through more than 158 because it is unlikely that the teeth of the gears will be synchronized, the drive having been under the control of gears 26, 20.

There is therefore a danger of the gears jamming, and possibly breaking. To avoid this jamming the tooth 30 and adjacent teeth are formed on a cantilevered part 32 of gear 27, the cantilever being formed by a slot 31 starting adjacent tooth 30 and passing beneath it. Gear 27 is formed of a resiliently deformable material, e.g. a platics material, which allows the free end of cantilever 32 to flex about its secured end. When the gear 27 is in the position of trying to engage with gear 21, the cantilever 32 may be pushed out of the way of unsynchronized gear teeth on gear 21 until synchronization is achieved. Thus the jamming action is avoided.

The cam 1 5 operates an alarm bell 33 (Fig.

1) through a striker 34, one end of which is interposed between the cam and a button 35 operating the microswitch, so that when the oven is turned off the alarm is also sounded.

While the invention has been described with specific reference to a microwave oven timer, it can be applied to any timer where it is desired to rotate the cam at different speeds.

Claims (10)

1. A timing device of the kind described having a first gear provided with gear teeth only over an arc of its circumference, said gear being rotatable with said cam, the said gear teeth moving into mesh with a first drive gear when the cam enters said certain arc, at least that tooth first engaging said first drive gear as the said gear teeth move into mesh therewith being spring-mounted so as to be resiliently movable to allow synchronisation with the first drive gear as the gears engage.

2. A timing device as claimed in claim 1, wherein said spring mounting is provided by undercutting the said tooth or teeth, the first gear being formed of resilient material.

3. A timing device as claimed in claim 2, wherein several teeth are undercut.

4. A timing device as claimed in any of claims 1 to 3, wherein said undercut forms a cantilever on which said teeth are formed.

5. A timing device as claimed in any of claims 1 to 4, wherein said first gear is formed of a resilient plastics material.

6. A timing device as claimed in any of claims 1 to 5, comprising a microwave oven timer in which cooking times are selectable by rotating said cam, shorter cooking times being selected by rotating said cam into said certain arc and longer cooking times are selectable by rotating said cam into said succeeding arc, said timing device including a motor driving said cam through gear train means including said first gear and said first drive gear so as to return said cam from said selected positions to a zero position.

7. A microwave oven timer as claimed in claim 6, wherein said first gear is integral with a second gear of said gear train forming a compound gear and a second drive gear is mounted on a common spindle with the said first drive gear, said second drive gear being permanently engaged in said second gear of the compound gear, and said first and second drive gears both being frictionally driven by said common spindle whereby one or the other drive gear may slip relative to the spin dle.

8. A microwave oven timer as claimed in claim 7, wherein each of said first and second drive gears is carried on a split collar secured to the spindle by a spring clip.

9. A microwave oven timer as claimed in any of claims 6 to 8, wherein said cam operates a microswitch which energizes both an oven and said motor when said cam is in any position other than said zero position and said gear train causes said cam to rotate at said other speed until 5 minutes of cooking time remain, and at said one speed, which is slower than said other speed, thereafter.

10. A microwave oven timer substantially as described hereinbefore with reference to the accompanying drawings.