What types of mechanism and drive system change motion, and how does a belt or chain drive work?
Mechanisms and drive systems: the four types of motion, belt and chain drives, and calculating the velocity ratio and output speed of a pulley drive.
An SQA National 5 Engineering Science answer on mechanisms and drive systems, covering the four types of motion, belt and chain drives and where each is used, and calculating the velocity ratio and output speed of a pulley system from the pulley diameters.
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What this key area is asking
The SQA wants you to recognise the four types of motion, know how belt and chain drives transmit rotation, and calculate the velocity ratio and output speed of a pulley drive from the pulley diameters.
Types of motion
A mechanism is any device that converts an input motion or force into a useful output. A bicycle, for example, converts the rider's reciprocating leg motion into rotary motion of the wheels.
Belt and chain drives
The two differ in their grip:
- A belt can slip if overloaded. This can be a safety advantage (it gives way rather than breaking the machine), but it means the drive is not perfectly exact. Belts run quietly and need little maintenance.
- A chain does not slip because the chain links engage the sprocket teeth, so it transmits motion exactly and can carry larger forces, as on a bicycle. Chains are noisier and need lubrication.
Belt and chain drives also let the two shafts be some distance apart, which gears cannot easily do.
Velocity ratio of a pulley drive
This works just like the gear ratio, but uses pulley diameters instead of numbers of teeth. A large driven pulley driven by a small driver pulley gives a high velocity ratio, so the output turns slowly but with more turning force. Unlike directly meshing gears, a simple open belt drive turns both pulleys in the same direction.
Why drive systems matter
Belt, chain and gear drives are how an engineer gets power from a motor to where it is needed, while changing the speed to suit the job. Choosing between them - belt for quiet, slip-protected drives; chain for exact, high-force drives; gears for compact, exact drives - is a typical design decision in the assignment.
Try this
Q1. Name the type of motion of a swinging pendulum. [1 mark]
- Cue. Oscillating motion.
Q2. A driver pulley is and a driven pulley is in diameter. State the velocity ratio. [1 mark]
- Cue. , i.e. .
Q3. State one advantage of a chain drive over a belt drive. [1 mark]
- Cue. A chain does not slip, so it transmits motion exactly and can carry larger forces.
Exam-style practice questions
Practice questions written in the style of SQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
SQA N5 style3 marksA belt drive has a driver pulley of diameter 40 mm and a driven pulley of diameter 120 mm. Calculate the velocity ratio.Show worked answer →
Use the velocity ratio relationship for a pulley drive.
Relationship: .
Substitution: , i.e. .
Markers reward driven diameter over driver diameter, correct substitution, and stating the ratio (3 or 3:1). The larger driven pulley turns more slowly than the driver.
SQA N5 style3 marksA driver pulley turns at 1500 rev/min and the pulley system has a velocity ratio of 5:1. Calculate the output speed.Show worked answer →
A velocity ratio of 5:1 means the output turns five times slower than the input.
Relationship: .
Substitution: .
Markers reward dividing the input speed by the velocity ratio and giving the answer with its unit.
Related dot points
- Gear systems and the gear ratio, calculating output speed from the gear ratio, and how gearing trades speed for torque.
An SQA National 5 Engineering Science answer on gear systems, covering the gear ratio as the ratio of driven to driver teeth, calculating output speed, how a gear train trades rotational speed for turning force (torque), and the direction reversal between meshing gears.
- Mechanical advantage, velocity ratio and efficiency of a mechanism, including calculating each and relating efficiency to wasted energy.
An SQA National 5 Engineering Science answer on mechanical advantage, velocity ratio and efficiency, covering mechanical advantage as load over effort, velocity ratio, the percentage efficiency relationship, and why a real machine's efficiency is always below 100% because of friction.
- Work done by a force, mechanical power as work done per second, and the relationships work equals force times distance and power equals work over time.
An SQA National 5 Engineering Science answer on work, energy and power, covering work done as force times distance, energy transferred equal to work done, mechanical power as work done per second, and the relationships needed to calculate each in an engineering context.
- Forces, the difference between mass and weight, the weight relationship W equals mg, and the force-mass-acceleration relationship F equals ma.
An SQA National 5 Engineering Science answer on forces, covering the difference between mass and weight, the weight relationship W equals mg, the force-mass-acceleration relationship F equals ma, and balanced and unbalanced forces in an engineering context.
Sources & how we know this
- SQA National 5 Engineering Science Course Specification — SQA (2017)
- SQA Engineering Science Data Booklet National 4/5 — SQA (2017)