Relay valve

Key takeaways

Relay valve belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Relay valve to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Relay valve from memory before moving on to harder problems.

Reference excerpt

A relay valve is an air-operated valve typically used in air brake systems to remotely control the brakes at the rear of a heavy truck or semi-trailer in a tractor-trailer combination. Relay valves are necessary in heavy trucks in order to speed-up rear-brake application and release, since air takes longer to travel to the rear of the vehicle than the front of the vehicle, where the front service brakes, foot-valve, parking-control valve, and trailer-supply valve (if applicable) are located. Without relay valves, it would take too long for sufficient air to travel from the brake pedal valve to the rear of the truck or trailer in order to apply the rear service brakes concurrently with the front service brakes, resulting in a condition known as brake lag. To correct this condition on a long-wheel-base vehicle, a relay valve is installed near the rear service brake chambers. In tractors as well as straight-trucks, a remote air-supply is provided in the form of a large diameter pipe connected between the primary reservoir and the relay valve for remote service brake application. In a truck’s air brake system, relay valves get a signal when a driver presses the pedal, which then opens the valve and allows air to enter the brake chamber via air inlet. The diaphragm gets pushed, then the rod, then the slack adjuster which twists to turn the brake camshaft. Next, it moves the disc, wedge or s-cam, which pushes the brake shoes and lining, creating friction. This friction slows and eventually stops the brake drum’s turning, which stops the wheel.

Trailers In trailers, this remote air-supply is in the form of a tank, which is charged whenever the emergency brakes are released via the red trailer-supply valve on the dashboard. In a dual-circuit air brake system, this tank actually receives its air from both the primary and secondary reservoirs of the tractor; the air from both of these reservoirs is merged via a two-way check valve. The two-way check valve is a pneumatic device that has two inputs and one output; each input is connected to one these reservoirs. Only the air that is at the higher pressure is allowed to pass through to the check valve's output, which then passes through the tractor-protection valve, and then travels onward towards the trailer's air-tank and spring brake valve via the red trailer-supply line (a.k.a., the emergency line); this releases the trailer's emergency brakes (a.k.a. spring brakes). The tractor-protection valve is a device that prevents air from being lost from the tractor's braking system in the event of the air-lines becoming separated or broken. The tractor's air-lines connect to the trailer's air-lines via metal connectors known as gladhands. The merged air from both reservoirs of the tractor prevents air-loss from only one tractor braking circuit from causing the trailer's spring brakes to automatically apply. This gives the driver more control, and prevents the vehicle from grinding to a halt in an unsafe location, such as in the middle of an intersection.

Service brake relay valve With a service brake relay valve installed, the hose that connects to the primary delivery-port output of the foot-valve becomes a control-line (i.e., The air from the foot-valve “dead ends” at the relay valve's control-port.). Only low-volume air-signals are required to travel back and forth between the foot-valve's delivery port and the relay valve's control port; therefore, the air-volume supplied by the delivery port is now only a tiny-fraction of what otherwise would have been required had the relay valve not been installed. This reduces the delay between the application of the front and rear brakes to only a fraction of a second. When the driver depresses the brake pedal, a small amount of air momentarily opens the relay valve's supply port, which then directs air from the remote air-supply directly to the rear service brake chambers, and quickly applies the rear service brakes. The pressure delivered to the service brake chambers in this manner will equal the control-pressure delivered by the foot-valve to the relay valve. When the driver partially or fully releases the brake pedal, the control-pressure delivered by the foot-valve decreases; this causes the relay valve's supply port to close, and its exhaust port to momentarily open, thus preventing a pneumatic short-circuit from occurring while the air exhausts from all rear service brake chambers. In order to control the trailer service brakes, the merged outputs (i.e., merged via 2 two-way check valves connected in-series to give three inputs) of the foot-valve and trailer-hand-valve (if applicable) are directed through the tractor-protection valve, and onward towards the trailer relay valve via the blue service line. In tractors that are not equipped with a trailer hand valve, only the merged outputs of the foot-valve (i.e., via a single two-way check valve) are directed towards the trailer relay valve; however, the fact that the foot-valve's delivery-port outputs are still merged enables the trailer's service brakes to still be controlled even if there is failure within one braking circuit of the tractor.

… excerpt ends here. Continue reading the full article.

Worked examples

Example 1 — a first encounter with Relay valve

Start with the simplest possible case. Write down what Relay valve claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Relay valve before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Relay valve ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Relay valve

In research: Relay valve appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Relay valve in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Relay valve is common in secondary-school and first-year university syllabi. It links to neighbouring topics Valves, so understanding it makes those chapters shorter.
In everyday life: Look for Relay valve outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

How to study Relay valve in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Relay valve means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Relay valve out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Relay valve in simple terms?

A relay valve is an air-operated valve typically used in air brake systems to remotely control the brakes at the rear of a heavy truck or semi-trailer in a tractor-trailer combination. Relay valves are necessary in heavy trucks in order to speed-up rear-brake application and release, since air take…

Why does Relay valve matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Relay valve?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Relay valve.