The Diesel Technicians Step-by-Step Guide to Hydraulics Schematics

Diesel technicians attempting to make repairs on a piece of construction equipment or farming equipment’s hydraulic system face some difficult challenges.  This blog post will attempt to break down the components of a hydraulic system, how they work, and help give a basic understanding of how to read a hydraulic schematic.

 What is a Hydraulic System?

Basically, hydraulic systems work by using force created by compressed fluid. There is a reservoir that is used to store non-pressurized hydraulic fluid, a motor pumps this fluid out during the piston’s up-stroke. As the piston is pushed downward the oil is moved into the hydraulic cylinder which increases pressure. This pressurized fluid creates mechanical force used to lift the load.

Understanding Hydraulics

Tip: Use caution when working with hydraulic systems. They are highly pressurized (2,000-3,000 PSI) and reach extreme heat so be sure pressure is relieved and allow it to cool before changing lines, connections, filters or fittings. 

Hydraulic Systems have a few basic components:

  • Fluid (Normally Hydraulic oil)
  • Reservoir – to hold the fluid
  • Pump – to force the fluid through the system
  • Valves – control the flow
  • Motor / actuator
  • Cylinder
  • Hose
  • Filter
  • Valves

There are 2 types of hydraulic valves:

  • Directional Control Valves: determines the flow path of the hydraulic oil
  • Pressure Relief Valve: these valves open whenever the system pressure exceeds the set value, which prevents pressure overloads and limit output force. When the valve opens, oil flows back into the reservoir.

Reading a Hydraulic Schematic for Beginners 

Click the button below to download a full guide that breaks down the fluid power symbols and electrical symbols.

Having a basic understanding of the hydraulic system components, and how to read them on a schematic, are essential to making repairs. To make it less overwhelming, let's break down the basic components of the system.

Hydraulic Reservoirs

Every hydraulic storage reservoir will have at least two connecting lines. The reservoir is the only component to be displayed more than once in a schematic.

Hydraulic Pumps

In a hydraulic pump schematic, the triangle always points in the direction that the oil will flow. If the pump is reversible or pumps in multiple directions, it will have two triangles that point in opposite directions which indicates that oil can flow in either direction. An arrow through the pump means that it is a variable displacement pump.

Hydraulic Motors

Unlike a hydraulic pump you will notice that the triangles point inward. This to show the oil flowing into the motor. Again, one triangle is a non-reversible motor, whereas an image with two triangles is a reversible motor. An arrow through the motor means that it is a variable speed motor.

Hydraulic Valves

Valve Envelopes: On Valve Envelopes you will see this broken down by squares indicating each position for the valve to be in. 

Control Valves: These indicate flow to pressurized passages or return to tank passages that hydraulic oil will flow.

Valve Ports: These are the places where hydraulic flow enters or leaves the valve referred to as inlet and outlets 

Check Valves (bottom right quadrant):  The basic symbol indicates a free-flowing direction and a non-flowing direction of oil.  Spring loaded check valves hold a ball against a valve seat to keep flow from returning to the circuit.  Shuttle valves allow two flow sources to be connected into one branch of the hydraulic circuit. Pilot controlled check valves let flow in one direction but allows flow in the opposite direction if pilot pressure is applied at the point of the pilot pressure point on the valve. 

Hydraulic Cylinder

This rectangle represents the cylinder’s barrel. The rod and piston are displayed using a tee that is positioned into the rectangle.

 Performing Hydraulic System Diagnostics and Repairs

Hopefully this gives you a surface level understanding the hydraulic system. at Diesel Laptops our mission is to help give mechanics and equipment owners the tools they need to perform their own diagnostics and repairs on their equipment.

Many of our customers have expressed interest in learning more about how to work on these systems so we created a hands-on training program to teach them. It is broken into two classes: Hydraulics System Fundamentals and Hydraulic System Diagnostics. By the end of these classes, students will have a comprehensive understanding of hydraulic systems and will know how to properly diagnose hydraulic systems to get to the root of the failure allowing them to save time and money.

Check out all of our upcoming courses including aftertreatment systems, electrical diagnostics, and much more, by clicking here.

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