Linear Mechanism

Linear mechanisms are very interesting for the Meccano engineer, and they are widely used in industry. Perhaps their widest application is in the machine tool industry, but they are also used in conveyers, robotics and in the laboratory.

The range of Meccano components enables most of the commonly used mechanisms to be assembled with little difficulty and there are a number of ways to build a linear drive in Meccano. There are two elements involved, namely the guideway and the trolley, and the means to move the carriage. The following mechanisms can be built in Meccano:

  1. Crank and slider – Car piston, connecting rod and crankshaft
  2. Peg and slot – Shaping machine
  3. Cam and follower – Car valve gear
  4. Eccentric – Steam engine valve gear
  5. Rack and Pinion – Hand operated carriage movement on Lathe
  6. Screw Thread – vice, car jack, lathe headscrew
  7. Carriage and slider – Fork Lift truck, Fire escape, Designing machine

Method of location of sliding component:

Slide on rods

Slide in grooves

Locating by multiple wheels with or without flanges

Methods of actuation and example:

Lead screw – machine tool, vice

Wire or cord – tower crane or blocksetter trolley

Coaxial, wire in tubing – Bicycle brake

Hydraulic, oil actuated cylinders – earth moving equipment

Pneumatic, air actuated cylinders – machine tools and assembly line equipment

Solenoid – short travel motion eg. Latch

Rack and Pinion – Steam shovel, machine tool, drawbridge.

There are other considerations not at first apparent, such as how to prevent an over-run at each end of a sliders travel, how to stop at predetermined distances along the slide, how to link the mechanism to the driven element .

The diagram shows one type of Linear Drive which has been found to be most effective. It has enormous power, even though actuated only by a simple low voltage motor with built-in epicyclic gearbox. There is no backlash, and the drive is smooth and without any vibration or significant noise.

Construction begins by bolting 3” x 1” flat plates to two 12 ” angle girders. These are fixed to each other at one end by a transverse 1 ” girder to which one flange of the motor could be bolted. The motor is an Escap motor with built in gearbox and a mounting flange, but any suitable size motor with low output speed will be equally satisfactory. At the other a 1 ” flat plate is bolted to two vertical 1 ” angle girders, which are themselves bolted to the long girders using corner brackets for extra rigidity. A 1 ” double angle strip joins the two side plates, and the motor flange can be attached to this as well.

The trolley is built up from two 1 ” double angle strips firmly bolted back to back with a central collar on a ” bolt separating them. 1” corner brackets are included to provide an attachment for a drag rod to the actuating mechanism. ” pulleys are located on ” bolts.

The trolley slides freely on 11 ” rods which are held at the motor end in threaded couplings fixed to the long girders. Their other ends are located in the end 1 ” x 1 ” flat plate.

A 7/8” bevel gear is fixed to the motor output shaft, and this meshes with a second similar bevel which is fixed to a 2” rod. A ” pinion is also fixed to this rod, and it meshes with a 50 tooth gear on a second 2”rod. Also fixed to this rod is a coupling which acts as a winding drum. Use a hollow nosed grub screw which lies flush with the coupling when tightened. Place ” washers on each side of the coupling to act as cheeks.

A tensioner is built at the end of the mechanism. This is an end bearing to which a 1” screwed rod is fixed by lock nuts. The screwed rod is placed in the middle upper hole of the end flat pate, with a nut turned onto the end of the screwed rod to await tightening later.

The actuating mechanism is 40“ length of 0.7mm wire. This is fixed at one end to a 2” rod held by collars in 1” triangular plates bolted to the front of the side plates. The wire is first passed through a wire ferrule, then round the rod and then back through the ferrule which is crushed with pliers and the short end cut. The wire then passes round the adjacent ” trolley pulley and back to be wrapped five times round the drum. Most important - ensure that the wire does not cross itself. It will be unruly at this stage, but will be made to settle down when tensioned. It then passes to the end pulley, back around the other trolley pulley and finally to the tensioner. Before it is passed round the tensioner bolt, it is passed through a second ferrule, and then round the bolt of the tensioner, and finally back through the ferrule. It can be pulled up fairly tight by hand, settling the coils on the drum, and running the ferrule almost up to the tensioner. The ferrule can now be crushed, and the wire tightened by the tensioner nut. Finger tight is more than adequate. Loosen the bevel drive to allow hand turning of the drum. There is no reason why firm inelastic cord cannot be used in place of the wire.

Now tighten the bevel and the motor can be activated and the trolley will slide smoothly up and down its guideways.

There are a number of ways to mount the mechanism. It can be fixed at the motor end by double arm cranks bolted to the outsides of the flat plates. The free trolley holes can then be attached to the actuating mechanism, such as a crank.

Another fixation method would be to fix the whole structure to the machine, and then link the trolley by a drag rod to the actuator.

Michael Adler

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