Electric Linear Actuator

This Instructable is about making a powerful linear actuator with typical household tools from the minimum of components from the hardware store - no milling or turning but there will be a little cutting and drilling!
This Instructable will guide you through the details of designing a linear actuator to suit your needs by using an electric screwdiver motor.

Heavy duty linear actuators are understandably expensive as precision mechanical design is required for reliable operation and can rarely be justified for a one-off home project.

A linear actuator is expected to pull or push a load in one plane (e.g in-and-out or up-and-down) so it is designed for a certain maximum load and a distance, known as the "throw".

The main difficulty in a project like this is the lack of machining capability for making reliable couplings to the drive and slider. The hexagon shaft of the screw-driver and a threaded tube from D.i.y furniture fittings solved these problems.

There are two parts to this Instructable in that we have an electrical part and a mechanical part.

:WARNING: :WARNING: :WARNING: :WARNING: :WARNING: :WARNING: :WARNING:

This device is capable of exerting a high force and should be operated with extreme caution.

An emergency "STOP" control is advised

and the mechanism should be fully enclosed if fitted in an accessable location.

A Hacksaw

A Drill & Drill Bits to suit the fixing screws e.g 2.5mm & 3mm

Screwdriver for the fixing screws

Two M6 spanners

A flat File or sand/glass paper for de-burring

The lead-screw
M6 threaded rod of 310 millimeters long

A guide frame 2 off 10 x 20 x 1.5mm un-equal right angle aluminium (530mm frame t.b.a) 3 off 10 x 20 x 1.5mm un-equal right angle aluminium (50mm cross braces and bracket) 2 off 10 x 20 x 1.5mm un-equal right angle aluminium (20mm spacers) total 1150mm

The moving part - the slider 1 off 10 x 10 square section aluminium (450mm long) 1 off 10 x 10 square section aluminium (12mm long) total 462mm

M6 nuts and washers and fixing screws: 1 off M6 threaded tube ( x25mm) 4 off M6 Nuts 2 off M6 Washers fixing screws total 14

A motor e.g electric screw-driver

Power supply

power switch

Change-over relay

Limit switches

Connecting wire

An electric motor - geared down

The electric screw-driver used here is nominally 2.4 volts and was run on two Ni-Cad re-chargable cells so a suitable power supply would be a Personal Computer P.S.U giving the option of 3.3 volt and 5 volt drive power. The current (Amperage) could be up to 6 Amps. so all components and wiring must be suitable.

As it happens, I decided to stay with the two Ni-Cad re-chargable cells as the operation was going to be intermittent and it meant that I could use the existing Charger!

The Frame secures everything relative to each other and takes the load strain; the moving part slides in the frame and is moved by a "traveling" nut on the lead-screw driven by an electric motor. The lead-screw is secured at the motor end and the "travelling" nut is secured to the moving part so that when the screw is turned it forces the moving part to follow the motion.
I am using the parts left-over from the grean-house actuator which are:

M6 threaded rod of 310 millimeters long

10 x 20 x 1.5mm un-equal right angle aluminium (1.2 metres long)

10 x 10 square section aluminium (1.0 metres long)

An electric motor - geared down

The dimensions of the parts required are all related to the "throw", meaning how far the "travelling" nut can move. there are three sections of a lead-screw i.e each end and the "travelling" nut.

Each fixed end of the threaded rod has a section which reduces the available screw thread; the useful thread length becomes 310mm -25 (rod) -40 (nuts and bearing = 245mm which is the effective travel distance.

The moving part has three sections; the connection to the "travelling" nut, the "throw" and extension: The "throw" is the lead-screw travel and the extension is the length required for stability plus reach to the object being driven.

I use half the "throw" distance in the frame for stability so so 245/2 = 122.5 then I add the lead-screw length to give 122.5 + 310 = 432.5mm minus the end-stop distance about 24mm so, about 405mm is the minimum and I am going to round it up to 450mm which gives the extra to make the attach. (310/2 = 160 *3 = 465mm)

The frame has to enclose the lead-screw, the support length and provide a mounting for the electric motor.

I am using 10 x 20 x 1.5mm off-cuts for cross-braces and holding the slider in the guide frame.

I am using 10 x 10 square section aluminium off-cuts for locating the relationship of the lead-screw to the 10 x 10 square section aluminium moving part.

So, parts required becomes:
1 off M6 threaded rod of 310 millimeters long 5 off M6 Nuts 2 off M6 Washers

2 off 10 x 20 x 1.5mm un-equal right angle aluminium (450mm frame t.b.a)

3 off 10 x 20 x 1.5mm un-equal right angle aluminium (50mm cross braces and bracket)

total 1260mm

1 off 10 x 10 square section aluminium (450mm long)

1 off 10 x 10 square section aluminium (12mm long)

total 462mm

fixing screws total 14

An electric motor - geared down

The electric motor has to be aligned and secured to the frame and this is done with two support rods: In this case, the motor diameter is 40mm meaning the centre is at 20mm which has to align with the lead-screw thread. The two support rods are screwed to the frame and "cradle" the electric motor so they are spaced to lower the centre-line.

2 off 10 x 10 square section aluminium, long enough to support the electric motor.

The lead-screw runs centerally in the 10mm channel of the frame and the support rods are fitted on the lower side of the frame: A little Math. using right angled triangles gives a adjacent side of 5mm and a hypotenues of 40/2 =20mm so 20 squared = 400 minus 5 squared (25) = 375 of which the sq. root is 19.365; the "cradle" width for 40mm diameter is twice this at 38.7 which will drop the electric motor centre line just so but beware that a tolerance of only +/- 0.5mm = 4 to 6mm difference!

The threaded rod needs a screw-driver slot cut and the first picture here shows how I managed to hold it secure for cutting with the Hacksaw.

a small slot for locating screws is made at each end of the threaded rod as shown in the second picture here and then mounted in the end of the Slider rod as shown in the third picture here.

The Aluminium sections are cut to length:

1 off 10 x 10 square section aluminium (450mm long)

plus a small guide

1 off 10 x 10 square section aluminium (12mm long)

which is used for the fixed end.

2 off 10 x 20 x 1.5mm un-equal right angle aluminium (450mm frame t.b.a)
2 off 10 x 20 x 1.5mm un-equal right angle aluminium (50mm cross braces)

including

2 off 10 x 20 x 1.5mm un-equal right angle aluminium (20mm spacers)

because the M6 nuts will need to rotate with the lead-screw so spacers are used to widen the slider chanel as part of the cross bracing.

A professional device would have a co-axial slider and lead screw:

the M6 threaded tube is fitted inside the slider
1 off M6 threaded tube ( x25mm)

1 off M6 threaded rod of 310 millimeters long

4 off M6 Nuts

2 off M6 Washers.

Two supports are added to mount the electric motor of 10 x 10 square section aluminium.

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In these pictures you can see the construction of the fixed end of the lead-screw.

The lead-screw is screwed into the slider and slid into the channel so that some 100mm thread is passed through the fixed guide for securing as described below

The small guide is fixed in the channel complete with the spacer pieces as the M6 nuts will need to rotate with the lead-screw. The small guide prevents the screw thread from cutting on the bearing area and I used a convenient piece of 8 x 8 square section aluminium inside the small guide as a bearing.

1 off 10 x 10 square section aluminium (12mm long)

The technique used here is to fix the lead-screw in place with a pair of lock-nuts.

If a nut is fitted to a screw and another is run up beside it the two can be made to stay in place by tightening one against the other.

The sequence on the lead-screw is 2 x M6 nuts, 1 x M6 washer, the fixed guide, 1 x M6 washer, 2 x M6 nuts.

The trick here is to run on the first two nuts and washer past the fixed guide then add the next washer and set the other two nuts at the end of the lead-screw, locked in place: To finish, the furthest two nuts are run back to touch the fixed guide then the furthest nut is held whilst the inner nut is locked towards it so leaving a little bit of end-play so the lead-screw can rotate freely.

The motor "cradle" pieces are screwed in place according to the calculations based on the diameter of the screw-driver body and the screw-driver bit is aligned into the slot of the lead-screw.

There are two tips I have to offer help to make the unit reliable:

1). There will inevitably be some small mis-alignment so I have found that it is best to fit some kind of sleeve over the meeting of the lead-screw and the screw driver bit; the other sleeve of some power lead or any plastic tubing will suffice.

2). A spring in the screw-driver fitment hexagon holds the bit in place against the lead-screw end; a suitable spring might be found in a thow-away soap dispenser.

Finally, a cross member is screwed to the slider which serves to hold the slider in the channel and conveniently activates the limit switches.

No linear actuator would be complete without limiting devices to stop over running either end of the "thow" and with an electric motor it is easy to fit micro-switches, which have the benefit of both normally open and normally closed contacts.

The first picture shows the micro-switches ready for wiring. Note: the micro-switches shown are the maximum limit switches so extra switches are required to stop the motor automatically at a different position.

The picture above shows the classic wiring of the Double pole / Double throw switch for reversing a D.C Electric Motor i.e two sets of independant change-over contacts.

The electric motor is connected to the common contacts, shown here as Black & Red, whilst the power is suppled to one pair of contacts, shown here as Blue & Brown, which are then cross-connected to the other pair of contacts, the Yellow and Blue wires.

In this case the cross-over wiring is replaced with the micro-switch normally closed contacts to prevent over-run and any extra limit switches are simply wired in series: On this switch the Brown wire is dis-placed to oppose the Blue.

When testing, make sure to check that the motor runs in the right direction and that the switches operate in the right sense!

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