The following animation shows the linkage of a windscreen wiper system – a crank-rocker mechanism that converts the rotation of the wiper motor into the oscillating movement of two synchronously running wiper arms. Crank radius, lever length and wipe angle can be changed in real time.
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Description of the Animation
The animation shows a tandem wiper system of the kind fitted to almost every passenger car. The wiper motor at the bottom left drives a crank (yellow). Via the drive link (blue), the lever on the driver’s side (green) is swung back and forth. The coupling rod (orange) transfers this swinging motion to the second wiper spindle, so that both wiper arms move in parallel and in sync.
The play button starts the motor. The checkbox displays the swept wiping area; the wipe angle is additionally shown in degrees.
Interactive Controls
The sliders allow the following parameters to be adjusted:
- Crank angle φ (0–360°): position of the motor crank for manual positioning
- Crank radius r (16–37): radius of the motor crank, determines the deflection
- Lever length h (54–84): length of the lever on the wiper spindle
The play/pause button starts and stops the rotation of the crank.
Physical Background
The wiper linkage is a four-bar mechanism – a crank-rocker. The motor crank rotates fully, whereas the lever on the wiper spindle can only oscillate. The drive link connects the two members.
The wipe angle, i.e. the swept range of the wiper arm, depends to a good approximation only on the ratio of crank radius to lever length:
\[ \psi = 2 \cdot \arcsin\left(\frac{r}{h}\right) \]
With the quantities:
- \( \psi \) – wipe angle
- \( r \) – crank radius at the motor
- \( h \) – lever length on the wiper spindle
A large crank radius or a short lever therefore increases the wipe angle. For the crank to rotate fully, \( r < h \) must always hold – otherwise the mechanism locks up in a toggle (dead-centre) position.
Despite the constant motor speed, the angular velocity of the wiper is not uniform: at the reversal points the arm is decelerated and accelerated again, while it is fastest in the middle of the wiping area. This non-uniform transmission is characteristic of crank-rocker mechanisms.
Together with the two equally long levers on the wiper spindles, the coupling rod forms a parallel crank linkage. Both wiper spindles therefore perform exactly the same swinging motion – the characteristic synchronous run of a tandem wiper system. The mounting angle of the arms on the splines of the spindle determines where the wipers come to rest in the park position.
Note: crank radius and lever length are shown enlarged compared with the real-world original. Since the wipe angle depends only on the ratio \( r/h \), the kinematics are unaffected by this.
A Brief History of the Windshield Wiper
In 1903, the American Mary Anderson was granted the first patent for a manually operated windshield wiper – she had observed streetcar drivers struggling with rain-covered windscreens during a trip to New York. By the 1920s, the wiper became standard equipment on most cars. Early systems used a single wiper arm driven by vacuum from the engine intake manifold, which meant the wipers slowed down when the driver accelerated. Electric motors replaced vacuum drives in the mid-20th century.
Today, tandem wiper systems with two synchronous arms – as shown in this animation – are the most common design on passenger cars. Other configurations include opposed systems (where both wipers swing outward from the centre), single-arm systems (common on rear windows and some European cars like the Mercedes W140), and the modern mono-arm wiper (sometimes called a “panoramic wiper”), which uses a single extra-long blade to cover the entire windshield.
Comparison with Other Four-Bar Linkages
The wiper mechanism shown here is one specific type of four-bar linkage. The German engineer Franz Grashof formulated a condition that determines how a four-bar linkage behaves: if the sum of the shortest and longest link is less than or equal to the sum of the two remaining links, at least one link can rotate fully (Grashof condition). Depending on which link is fixed (the “frame”), four-bar linkages are classified into distinct types:
- Crank-rocker (the wiper case): One link rotates fully (crank), the opposite link only oscillates (rocker). This is the type used in the animation.
- Double-crank (drag-link): Both links connected to the frame can rotate fully. Used, for example, in locomotive wheel couplings.
- Double-rocker: Neither link connected to the frame can rotate fully – both oscillate. The coupling link is the only member that can make a full revolution. Found in some types of rock crushers and agricultural machinery.
- Parallel crank: A special case where opposite links are equal in length. Both cranks rotate at the same angular velocity. This is exactly what the coupling rod in the wiper system achieves for the two wiper spindles.
If the Grashof condition is not met, no link can rotate fully, and the mechanism can only rock back and forth – a “triple-rocker” that must be driven by an external oscillating input.
Practical Applications
- Windscreen wiper systems: The most familiar everyday example. The crank-rocker converts continuous motor rotation into the oscillating sweep of the wiper blade. The non-uniform angular velocity is actually beneficial here: the wiper slows down at the reversal points, allowing the rubber blade to change direction smoothly without juddering.
- Vibrating screens and shaker conveyors: In mining and aggregate processing, crank-rocker mechanisms drive vibrating screens that separate material by grain size. The oscillating motion shakes the material across inclined screen surfaces, allowing finer particles to fall through while larger pieces travel onward.
- Textile machinery: Loom sleys – the frames that push the weft thread tightly into the fabric – are driven by crank-rocker mechanisms. The non-uniform speed provides a quick, firm beat-up followed by a slower return, which is ideal for producing tightly woven cloth.
- Agitators and mixers: In the chemical and food industries, reciprocating stirrers use crank-rocker drives to move paddles or blades back and forth through viscous liquids, ensuring thorough mixing without the vortex problems of continuous rotation.
- Prosthetics and rehabilitation devices: Four-bar linkages are used in artificial knee joints. The changing instantaneous centre of rotation of a four-bar mechanism can closely mimic the complex rolling-sliding motion of the natural knee, resulting in a more natural gait.
- Robotic grippers: Many parallel-jaw grippers in industrial robots use four-bar linkages to ensure that both jaws move symmetrically and apply equal gripping force, regardless of the size of the object being grasped.
