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IMPROVEMENTS IN ELECTRIC WRAP SPRING CLUTCHES.

1. INTRODUCTION.

To the best of our knowledge, electric wrap spring clutches have thus far only been designed to operate in one direction of rotation. Our new design operates in either direction, allowing it to be used with reversing motors and drives. Of the many applications foreseen, two will be summarised at this introductory stage.

1.1 REPLACEMENT OF VEHICLE DIFFERENTIALS.

To turn corners, most wheeled vehicles have differentials to cater for the different rotational speeds of the driving wheels. In contrast, tracked vehicles mostly de-clutch and brake their inner tracks, whilst driving their outer tracks. A similar approach is possible on wheeled vehicles, but without the need to brake one wheel. By replacing the differential with an overrunning clutch at each driving wheel, the inner wheel will transmit full torque through corners, and the outer wheel will simply overrun its clutch.

Why have overrunning clutches not appeared in vehicles thus far? We think because only unidirectional designs have been available, and all such designs have two major disadvantages:

Cannot transmit drive in reverse.
Cannot provide engine braking.

Both these restrictions disappear with our reversing design. As will be described, selective energising of the two solenoids per clutch, results in identical overrunning clutch performance in either direction of rotation, and with either side as the input or output.

Comparing the two approaches, differentials distribute torque equally until one wheel spins, at which time part or all torque is lost. Overrunning clutches will not distribute torque as evenly, but will ensure that the maximum possible torque is maintained across one or both wheels, when one spins, or both spin. The overrunning clutch approach is therefore arguably superior in slippery conditions, or when inner wheel lift occurs during cornering. We do not envisage that our clutch design will replace differentials generally. Rather, we think that a range of vehicles will emerge on which our approach will produce cost savings, and/or more reliable traction.

1.2 REGENERATIVE BRAKING AND ENERGY RECOVERY.

We foreshadowed our reversing overrunning clutch in our page IMPROVEMENTS IN ELECTRIC VEHICLE MOTORING AND REGENERATION CONTROL. In Section 4.3 therein, we deal with electric walk-behind golf buggies and observe that currently, most designs include unidirectional overrunning clutches to cater for cornering. Regeneration is not then possible, since the wheels cannot "drive" the motor, as is the prerequisite. The same restriction applies on any vehicles which use such unidirectional clutches.

Our new reversing design removes this restriction and opens the way for regenerative braking and energy return, on golf buggies and on light electric vehicles generally. Reversal of the overrunning direction is done by interchanging the energising of solenoids (2 solenoids per clutch, 1 clutch per driving wheel). Our control system does this automatically as the torque required at the wheels changes sign, with motor torque applying uphill and on the flat, and generator torque on downhill sections. For further details, refer to the above-mentioned page and section.

1.3 STATUS.

The following sections cover our concepts for the first prototype, noting that it has yet to be built and tested, and our explanation of the clutch's operation. As with other inventions in this series, we have lodged a Provisional Patent Application. We will be publishing results progressively, but will welcome early comments and/or expressions of interest, particularly from companies currently manufacturing wrap spring clutches. We anticipate that downstream designs will not only open the way for enhancing product features, but will also prove attractive by providing cost savings.

2. DESCRIPTION OF PLANNED PROTOTYPE DESIGN.

Our first prototype has been designed to transmit up to 440 lb.ins. (50Nm) torque, by 0.625" (16mm) shafting. The overall dimensions arrived at are 2.36" (60mm) O.D., and 5.43" (138mm) axial length. Each solenoid will require 750 to 900 turns of 30 AWG (0.010") dia. magnet wire, with two solenoids (one per clutch) energised in series across 12 volts. Energy consumption should not exceed 2 watts per solenoid.

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Steel Rotors 1 & 2 rotate concentrically, being axially aligned by Sintered Bearings 3 & 4, assembled to 1 & 2 respectively. Either side can be the input or output.
Aluminium Collars 5 locate steel Sleeves 6, concentrically with 1 & 2.
Threaded Pins 7 locate 5 & 6 and fix them to rotate rigidly with 1 & 2.
Rotors 1 & 2 are running fits in plastic Bearings 8, which also carry Solenoid Windings 9.
Bearings 8 are close fits in Cups 10.
Cups 10 assemble to plastic or aluminium Casing 11, and the final assembly is secured by End Caps 12 & 13, and sets of Screws 14 & 15.
Steel Control Rings 16 are close fitting but free to move axially a few mils on the spigots of Collars 5. The radial air gaps between these Control Rings and the I.D's. of Sleeves 6 are 0.010" (0.25mm).
The air gaps between the O.D's. of Sleeves 6 and the I.D's. of Cups 10 are likewise 0.010" (0.25mm).
The air gaps between journals on Rotors 1 & 2, and Cups 10 plus End Caps 12 & 13, are likewise 0.010" (0.25mm).
Spring 17 is of conventional 0.080 (2mm) square section wrapping clutch design and fit. It is wound with two diameters, which differ by 0.040 (1mm). The two sections have equal numbers of turns. The diameters are such that one half interferes on the O.D. of one Rotor, whilst the other half interferes on the I.D. of the opposing Sleeve. The Spring ends are bent 90 deg. axially, and engage in slots in the two Collars 5.
When both Solenoids are unenergised, Control Rings 16 float and the Rotors are disengaged (see later explanation in Section 3 below).
When either Solenoid is energised, the resulting magnetic flux follows the path shown by heavy lines, across 3 air gaps, also shown by heavy lines. Control Ring 16 in the magnetic circuit is then attracted to the adjacent Rotor shoulder and the frictional grip resulting is sufficient to wrap the Spring inwards or outwards, so as to lock to a Rotor or Sleeve, as required. The explanation of the clutch's operation is given in detail in Section 3.
It is to be noted that the following parts act together, and are STATIONARY - Bearings 8; Solenoid Windings 9; Cups 10; Casing 11; End Caps 12 & 13. These sections of the assembly "float", in effect, on the journals of the Rotors, via the Bearings 8.
Slots will be included in the Casing and rear shoulders of the Cups for outward passage of the connections to the Solenoids.
End Cap 13 depicts a typical means of fixing the unit to a motor or gearbox, with provision for securement of shafts by roll pins, keys or grub screws.
As shown on Sleeves 6, circumferential slots are included adjacent to the Control Rings to minimise unproductive flux. Four slots are planned, each 0.16" (4mm) wide by 1.18" (30mm) long, leaving four webs each 0.24" (6mm) in circumferential length.
The following mass production methods are envisaged:

Rotor 1, machined from bright M.S. bar.

Rotor 2 and Control Rings 16, machined from bright M.S. hollow bar.

Collars 5, diecast aluminium, machined.

Sleeves 6, thin wall seamless tube.

Cups 10 and End Caps 12 & 13, blanked and drawn.

It is to be noted that the foregoing outlines a "first-up" design of a prototype. Functional improvements and cost reductions will no doubt arise as the development proceeds.

3. EXPLANATION OF OPERATION.

SYMBOLS.

The following symbols will be used in the explanation of Figure 2 below:

A - Outer circumference of left-hand Rotor.

B - Inner circumference of left-hand Sleeve. A & B are rigidly connected.

C - Outer circumference of right-hand Rotor.

D - Inner circumference of right-hand Sleeve. C & D are rigidly connected.

T1, T2, T3, T4 - Directions of external torques applied.

SL - Left-hand Solenoid.

SR - Right-hand Solenoid.

RL - Left-hand Control Ring. Locks to A when SL energised.

RR - Right-hand Control Ring, Locks to C when SR energised.

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CONDITIONS APPLYING

RESULTING OUTPUT.
Both Solenoids off

When T1 applied

When T2 applied

WhenT3 applied

When T4 applied

 

 RL & RR float

B slips on Spring O.D

C slips on Spring I.D.

Spring I.D. slips on C

Spring O.D. slips inside B

RESULT. In all 4 torque directions, unit is de-clutched.

Only the small spring slip torque is transmitted.

Both Solenoids on

If torques applied

 Spring wraps in either direction and transmission drives as a solid shaft, in either direction.
SL energised, causing RL  to grip A

T1 then applied

 Left- hand spring half wraps inwards to A and locks to it, and right-hand spring half locks onto C.

RESULT. Torque T1 transmitted, or C overruns, if its speed exceeds the speed of A.
SR energised, causing RR to grip C

T2 then applied

 Right-hand spring half wraps outwards to D and locks to it, and left-hand spring half locks to the inside of B.

RESULT. Torque T2 transmitted, or B overruns, if its speed exceeds the speed of C.
SR energised, causing RR to grip C

T3 then applied

 Right-hand spring half wraps outwards to D and locks to it, the left-hand spring half having locked inside B.

RESULT. Torque T3 transmitted, or spring overruns inside B, if speed of C exceeds speed of B.
SL energised, causing RL to grip A

T4 then applied

 Left-hand spring half wraps inwards to A and locks to it, the right-hand spring half having locked to C.

RESULT. Torque T4 transmitted, or spring overruns on C, if speed of B exceeds speed of C.

This analysis bears out our claims regarding our clutch's function and benefits.

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