What it shows
Engines, compressors and pumps convert rotation to straight-line motion. A slider-crank gives a motion that is not quite sinusoidal, and a scotch yoke gives a perfect sine. The project builds both from plywood or acrylic and measures piston position against crank angle.
The science
In a slider-crank, a crank of radius r turns a connecting rod of length l that drives a slider. The position x = r cos θ + sqrt(l² minus r² sin² θ), which departs from a pure cosine when r/l is large. In a scotch yoke, a pin on the crank slides in a slot in the yoke, so x = r cos θ exactly. The yoke gives simple harmonic motion, but wears more because of sliding friction.
Materials
Cost: under Rs 1,500. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- 6 mm plywood or acrylic sheets
- Bolts, nuts and washers M4
- A 12 V geared motor or a hand crank
- Metal rods or bolts as pins
- A ruler glued along the slider
- A protractor disc fixed to the crank
- A phone for slow-motion video
Safety
For this project: Wear eye protection when cutting and drilling, and an adult must use the saw and drill. Keep fingers clear of the moving links.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
10 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Choose r = 3 cm for the crank and l = 12 cm for the rod for the slider-crank, and draw all the parts to full scale.
Cut the base, crank disc, connecting rod, slider and the guide for the slider.
Drill pivot holes exactly at the chosen distances using a punch mark and a small pilot drill first.
Assemble the slider-crank with loose bolts so that links turn freely, and fix the ruler on the slider track.
Build the scotch yoke with a slot of width slightly larger than the pin and a similar guide.
Rotate the crank by hand in 30 degree steps and record slider position from the ruler for both mechanisms.
Calculate theoretical positions from the formulae in a spreadsheet and compare with measured values.
Plot position against angle for both and see how the slider-crank is not symmetrical.
Run each with the motor at slow speed and film at slow motion to estimate the maximum speed of the slider.
Write the report with drawings, calculations, graphs and applications.
What you should see
The scotch yoke gives a symmetric sine curve, and the slider-crank gives a curve that is flatter at one end, with measured positions within about 2 mm of the calculated ones.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1What does the connecting rod length change?
A longer rod makes the slider-crank motion closer to a pure sine.
2Where is the slider-crank used?
In petrol and diesel engines, compressors and steam engines.
3Why does a scotch yoke wear faster?
The pin slides in the slot under load, and sliding friction is greater than the rolling or pivot friction in a crank.
4What is the stroke?
The distance between the two extreme positions, equal to twice the crank radius.
Take it further
Add a spring-loaded pen to draw the displacement graph on a moving paper strip, a mechanical oscilloscope.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
Free ebooks for this project
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Mini and Major Project Guide
For diploma and BTech students: choose, plan, build, document and present
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Tools, Materials and Safety for Model Making
Everything you need to build neat, strong and safe science models