H&P Lecture notes

Hydraulic and Pneumatic Systems : 2005/2006 I. Hydraulic and Pneumatic Systems 1 Hydraulic and Pneumatic Systems

Power train : Hydraulic and Pneumatic Systems 2 Power train Mechanical power transmission: Gears Belt drive Friction drive Rigid couplings Clutches Prime mover AC Motor DC Motor Diesel Engine Otto Engine Power transmission system Machine (linear or rotational motion) Mi, ωi M0, ω0 F0, v0 Properties: Continuously variable drive is difficult The relative spatial position of prime mover is fixed If the motor is electrical (DC motor or AC motor with variable frequency), then the rotational speed can be continuously changed but they are expensive

Hydraulic power transmission : 2005/2006 I. Hydraulic and Pneumatic Systems 3 Hydraulic power transmission Hydraulic power transmission: Hydro = water, aulos = pipe The means of power transmission is a liquid (pneumatic  gas) Hydrodynamic power transmission: Turbo pump and turbine Power transmission by kinetic energy of the fluid Still the relative spatial position is fixed Compact units Hydrostatic power transmission: Positive displacement pump Creates high pressure and through a transmission line and control elements this pressure drives an actuator (linear or rotational) The relative spatial position is arbitrary but should not be very large because of losses (< 50 m) A continuously variable transmission is possible Most of this lecture will be about hydrostatic systems (in common language it is also called simply hydraulics)

Hydrostatic vs hydrodynamic systems : 2005/2006 I. Hydraulic and Pneumatic Systems 4 Hydrostatic vs hydrodynamic systems Roughly speaking: P = Dp·Q Large Q, small Dp  hydrodynamic transmission Large Dp, small Q  hydrostatic transmission. But there is no general rule, depends on the task. Generally larger than 300 kW power hydrodynamic is more favourable. But for soft operation (starting of large masses) hydrodynamic is used for smaller powers either. Linear movement against large forces: hydrostatic Linear movement and stopping in exact position: also hydrostatic Power density P [kW] 100 200 300 400 Hydrostat. Hydrodyn.

Structure of a hydrostatic drive : 2005/2006 I. Hydraulic and Pneumatic Systems 5 Structure of a hydrostatic drive Aggregate Control elements Actuator These components and their interaction is the subject of this semester Valves, determining the path, pressure, flow rate of the working fluid Elements doing work Linear Rotational Swinging Pump, motor Fluid reservoir Pressure relief valve Filter Piping

A typical hydraulic system : 2005/2006 I. Hydraulic and Pneumatic Systems 6 A typical hydraulic system 1 – pump 2 – oil tank 3 – flow control valve 4 – pressure relief valve 5 – hydraulic cylinder 6 – directional control valve 7 – throttle valve

Advantages of hydrostatic drives : 2005/2006 I. Hydraulic and Pneumatic Systems 7 Advantages of hydrostatic drives Simple method to create linear movements Creation of large forces and torques, high energy density Continuously variable movement of the actuator Simple turnaround of the direction of the movement, starting possible under full load from rest Low delay, small time constant because of low inertia Simple overload protection (no damage in case of overload) Simple monitoring of load by measuring pressure Arbitrary positioning of prime mover and actuator Large power density (relatively small mass for a given power compared to electrical and mechanical drives) Robust (insensitive against environmental influences)

Disadvantages of hydrostatic drives : 2005/2006 I. Hydraulic and Pneumatic Systems 8 Disadvantages of hydrostatic drives Working fluid is necessary (leakage problems, filtering, etc.) It is not economic for large distances

Hydraulic fluids - tasks : 2005/2006 I. Hydraulic and Pneumatic Systems 9 Hydraulic fluids - tasks They have the following primary tasks: Power transmission (pressure and motion transmission) Signal transmission for control Secondary tasks: Lubrication of rotating and translating components to avoid friction and wear Heat transport, away from the location of heat generation, usually into the reservoir Transport of particles to the filter Protection of surfaces from chemical attack, especially corrosion

Hydraulic fluids - requirements : 2005/2006 I. Hydraulic and Pneumatic Systems 10 Hydraulic fluids - requirements Functional Good lubrication characteristics Viscosity should not depend strongly on temperature and pressure Good heat conductivity Low heat expansion coefficient Large elasticity modulus Economic Low price Slow aging and thermal and chemical stability  long life cycle

Hydraulic fluids - requirements (contd.) : 2005/2006 I. Hydraulic and Pneumatic Systems 11 Hydraulic fluids - requirements (contd.) Safety High flash point or in certain cases not inflammable at all Chemically neutral (not aggressive at all against all materials it touches) Low air dissolving capability, not inclined to foam formation Environmental friendliness No environmental harm No toxic effect

Hydraulic fluid types : 2005/2006 I. Hydraulic and Pneumatic Systems 12 Hydraulic fluid types Water (3%) Mineral oils (75%) Not inflammable fluids (9%) Biologically degradable fluids (13%) Electrorheological fluids (in development)

Hydraulic fluid types (contd.) : 2005/2006 I. Hydraulic and Pneumatic Systems 13 Hydraulic fluid types (contd.) - Clear water - Water with additives Oldest fluid but nowadays there is a renaissance Used where there is an explosion or fire danger or hygienic problem: Food and pharmaceutical industry, textile industry, mining 1. Water: Advantages: No environmental pollution No disposal effort Cheap No fire or explosion danger Available everywhere 4 times larger heat conduction coefficient than mineral oils 2 times higher compression module than mineral oils Viscosity does not depend strongly on temperature

Hydraulic fluid types (contd.) : 2005/2006 I. Hydraulic and Pneumatic Systems 14 Hydraulic fluid types (contd.) 1. Water: Disadvantages: Bad lubrication characteristics Low viscosity (problem of sealing, but has good sides: low energy losses) Corrosion danger Cavitation danger (relatively high vapour pressure) Limited temperature interval of applicability (freezing, evaporating) Consequences: needs low tolerances and very good materials (plastics, ceramics, stainless steel)  components are expensive

Hydraulic fluid types (contd.) : 2005/2006 I. Hydraulic and Pneumatic Systems 15 Hydraulic fluid types (contd.) - Without additives - With additives „Conventional” use, stationary hydraulics Always mixtures of different oils, often with additives Additives: decrease corrosion increase life duration improve temperature dependence of viscosity improve particle transport 2. Mineral oil: Advantages: Good lubrication High viscosity (good for sealing, bad for losses) Cheap Disadvantages: Inflammable Environmental pollution

Hydraulic fluid types (contd.) : 2005/2006 I. Hydraulic and Pneumatic Systems 16 Hydraulic fluid types (contd.) - Contains water - Does not contain water mines, airplane production, casting, rolling, where there is explosion and fire danger Water-oil emulsions (oil synthetic) or water-free synthetic liquids 3. Not inflammable fluids: Disadvantages: Higher density, higher losses, more inclination to cavitation Limited operational temperature < 55 °C Worse lubrication characteristics, reduction of maximum load Worse de-aeration characteristics Sometimes chemically aggressive against sealing materials

Hydraulic fluid types (contd.) : 2005/2006 I. Hydraulic and Pneumatic Systems 17 Hydraulic fluid types (contd.) - Natural - Synthetic Environmental protection, water protection Agricultural machines Mobile hydraulics Characteristics similar to mineral oils but much more expensive. If the trend continues its usage expands, price will drop. 4. Biologically degradable fluids:

Properties of hydraulic fluids : 2005/2006 I. Hydraulic and Pneumatic Systems 18 Properties of hydraulic fluids Viscosity: well-known Temperature dependence Ubbelohde-Walther: c, m, Kv are constants, T is in K  log-log scale Vogel-Cameron: A, B, C are constants, t is in °C

Properties of hydraulic fluids (contd.) : 2005/2006 I. Hydraulic and Pneumatic Systems 19 Properties of hydraulic fluids (contd.) Pressure dependence of viscosity 0, 0 viscosity at atmospheric pressure

Properties of hydraulic fluids (contd.) : 2005/2006 I. Hydraulic and Pneumatic Systems 20 Properties of hydraulic fluids (contd.) Density dependence on pressure: like Hooke’s law, K is the compressibility K is not a constant but depends on pressure itself effective K is also influenced by: Air content Flexibility of the pipe Temperature dependence of density is small

Hydraulic Fluids : 2005/2006 I. Hydraulic and Pneumatic Systems 21 Hydraulic Fluids Sucking air with the pump happens but is by proper installation avoidable. The oil is quickly into solution during the increasing pressure. Air bubbles come to oil mostly so that with decreasing pressure the air „goes out of solution”.  - dissolving coefficient at normal pressure At normal pressure Va=Vf . At high pressure, the volume of the dissolved air is much more than the volume of the liquid. When the pressure drops the air leaves the solution suddenly but the dissolution happens gradually. Air content in oil is harmful.

Hydraulic Fluids : 2005/2006 I. Hydraulic and Pneumatic Systems 22 Hydraulic Fluids Sudden, jerky movements, oscillation, noise Late switching Reduced heat conduction Accelerated aging of the liquid, disintegration of oil molecules Cavitation erosion Problems with air content: Kl : liquid compressibility Vf : volume of liquid Va0 : volume of gas in normal state p0 : normal pressure p : p under investigation

Hydraulic Fluids : 2005/2006 I. Hydraulic and Pneumatic Systems 23 Hydraulic Fluids The manufacturer specifies the characteristics of the required liquid and the duration of usage. Before filling in the new oil, the rig has to be washed with oil. Never mix old and new oil!

Calculation basics : 2005/2006 I. Hydraulic and Pneumatic Systems 24 Calculation basics

Calculation basics : 2005/2006 I. Hydraulic and Pneumatic Systems 25 Calculation basics Flow resistance:

Calculation basics : 2005/2006 I. Hydraulic and Pneumatic Systems 26 Calculation basics laminar Calculation basics: turbulent For a straight, stiff pipe: If the two cross sections are not the same then:

Calculation basics : 2005/2006 I. Hydraulic and Pneumatic Systems 27 Calculation basics Usually the function = (Re) looks like the following: Calculation basics: Practically:

Calculation basics : 2005/2006 I. Hydraulic and Pneumatic Systems 28 Calculation basics On this basis we can define two hydraulic resistances: Calculation basics: Depends on viscosity Does not depend on viscosity

Calculation basics : 2005/2006 I. Hydraulic and Pneumatic Systems 29 Calculation basics For a parallel circuit: For elbows, sudden expansions, T-pieces, etc. values are given as a function of Re, roughness and geometric parameters For a series circuit: Three different coefficients are used to express pressure loss: Gh: Hydraulic admittance

Leakage losses : 2005/2006 I. Hydraulic and Pneumatic Systems 30 Leakage losses Leakage losses: External losses Internal losses Occur always when components move relative to each other They reduce efficiency In case of external leakages there is environmental damage and the lost fluid has to be refilled. External losses can be avoided by careful design and maintenance. Internal losses cannot be avoided.

Leakage losses : 2005/2006 I. Hydraulic and Pneumatic Systems 31 Leakage losses

Leakage losses : 2005/2006 I. Hydraulic and Pneumatic Systems 32 Leakage losses

Leakage losses : 2005/2006 I. Hydraulic and Pneumatic Systems 33 Leakage losses the eccentricity increases the leakage flow by a factor of 2,5 if e increases to the limit QL ~ s3m ! Because of the large p, there are large temperature differences along l. Medium viscosity has to be substituted. In addition there is a Couette flow – dragged flow, which increases or decreases the leakage

Hydraulic capacity and inductivity : 2005/2006 I. Hydraulic and Pneumatic Systems 34 Hydraulic capacity and inductivity All the things discussed so far referred to steady processes. In practice, however, very often unsteady processes are encountered: starting, stopping, change of load, change of direction of motion, etc. In these cases the compressibility of the fluid and the pipes, and the inertia of the fluid have to be taken into consideration. Nonlinear function. It can be locally linearized and: Hydraulic capacity:

Hydraulic capacity and inductivity : 2005/2006 I. Hydraulic and Pneumatic Systems 35 Hydraulic capacity and inductivity Hydraulic capacity: The capacity has three parts: The capacitive flow rate: K compression module Cpipe is negligible if the pipe is made of metal Cpipe is not negligible if the pipe is flexible.

Hydraulic capacity and inductivity : 2005/2006 I. Hydraulic and Pneumatic Systems 36 Hydraulic capacity and inductivity Ltotal = Lh + Lsol , where Lsol is the inertia of solid parts. Hydraulic inductivity:

Hydraulic Accumulators : 2005/2006 I. Hydraulic and Pneumatic Systems 37 Hydraulic Accumulators Constructions and tasks in the hydraulic system Tasks: The hydropneumatic accumulators perform different tasks in the hydraulic systems, e.g.: Constructions reserve energy store fluid emergency operate force compensating damp mechanical shocks absorb pressure oscillations compensate leakage losses springs in vehicles recover of braking energy stabilize pressure compensate volumetric flow rate (expansion reservoir) With spring With weight With gas (hydropneumatic accumulator) Separating part between gas and fluid Piston Bladder Membrane

Hydraulic Accumulators : 2005/2006 I. Hydraulic and Pneumatic Systems 38 Hydraulic Accumulators Constructions With bladder With piston With membrane above welded below screwed

Hydraulic Accumulators : 2005/2006 I. Hydraulic and Pneumatic Systems 39 Hydraulic Accumulators Working states of hydroaccumulators with bladder: This installation is practically a bladder filled with gas and placed in a tank made out of steal. The bladder is filled with carbon dioxide (gas pressure). At the starting of the pump the fluid flows in the tank and compresses the gas. When required (if there is a high enough pressure difference) the fluid flows very quickly back in the system. Requirements on the system side: - locks both in the T and P lines, controlled release valves, juncture for pressure manometer (mostly built with the hydroaccumulator together), throw back valve in the P line. Fluid flows out Fluid flows in Hydroaccumulator with pre-stressed bladder pressureless, without pre-stress

Hydraulic Accumulators : 2005/2006 I. Hydraulic and Pneumatic Systems 40 Hydraulic Accumulators Construction Membrane Bladder Piston

Big pictures : 2005/2006 I. Hydraulic and Pneumatic Systems 41 Big pictures End of normal presentation Beginning of big pictures

Hydraulic Systems : 2005/2006 I. Hydraulic and Pneumatic Systems 42 Hydraulic Systems

Hydraulic Systems : 2005/2006 I. Hydraulic and Pneumatic Systems 43 Hydraulic Systems

Hydraulic Systems : 2005/2006 I. Hydraulic and Pneumatic Systems 44 Hydraulic Systems Continuity

Hydraulic Systems : 2005/2006 I. Hydraulic and Pneumatic Systems 45 Hydraulic Systems Pascal’s law

Hydraulic Systems : 2005/2006 I. Hydraulic and Pneumatic Systems 46 Hydraulic Systems Bernoulli equation

Hydraulic Systems : 2005/2006 I. Hydraulic and Pneumatic Systems 47 Hydraulic Systems Flow resistance

Hydraulic Systems : 2005/2006 I. Hydraulic and Pneumatic Systems 48 Hydraulic Systems Viscosity over temperature Temperature [C°] Viscosity [mm2/s]

Hydraulic Systems : 2005/2006 I. Hydraulic and Pneumatic Systems 49 Hydraulic Systems Accumulators: With spring With weight With gas (hydropneumatic accumulator) Separating part between gas and fluid Piston Bladder Membrane

Hydraulic Systems : 2005/2006 I. Hydraulic and Pneumatic Systems 50 Hydraulic Systems Accumulators:

Hydraulic Systems : 2005/2006 I. Hydraulic and Pneumatic Systems 51 Hydraulic Systems Accumulators: Gas filling screw Tank Membrane Valve-disc Juncture for hydraulic system

Hydraulic Systems : 2005/2006 I. Hydraulic and Pneumatic Systems 52 Hydraulic Systems Accumulators:

Hydraulic Systems : 2005/2006 I. Hydraulic and Pneumatic Systems 53 Hydraulic Systems Accumulators:

Hydraulic Systems : 2005/2006 I. Hydraulic and Pneumatic Systems 54 Hydraulic Systems Accumulators:

Hydraulic Systems : 2005/2006 I. Hydraulic and Pneumatic Systems 55 Hydraulic Systems Accumulators with bladder:

Hydraulic Systems : 2005/2006 I. Hydraulic and Pneumatic Systems 56 Hydraulic Systems Accumulators with membrane:

Hydraulic Systems : 2005/2006 I. Hydraulic and Pneumatic Systems 57 Hydraulic Systems Accumulators with piston:

Hydraulic Systems : 2005/2006 I. Hydraulic and Pneumatic Systems 58 Hydraulic Systems Typical hydraulic system:

Hydraulic Systems : 2005/2006 I. Hydraulic and Pneumatic Systems 59 Hydraulic Systems Pressure reservoirs = Accumulators Serve three purposes: damping of pressure and volumetric flow rate oscillations, supplying the flow rate at variable demand, hydropneumatic spring. They use the compressibility of a gas but the gas and liquid surface may not touch because then the gas will be dissolved in the liquid. Three constructions: Piston Bladder (bag) Membrane gas liquid a. b. c.