Alex BrandDue 11/26/2018
Manufacturing Methods – Michael Prechtl Hydraulic cylinders
I. Introduction: Hydraulic cylinders are a means of fluid power, more specifically a liquidous fluid. Oil is used in these systems to direct the movement of the components of a hydraulic cylinder. They are implemented in many systems varying greatly in size, force output, and working conditions. Because of these variations, hydraulic systems are a convenient solution due to the availability of mass produced standard sized cylinders. In contrast to hydraulic systems, mechanical means of generating force are often large, hazardous, and create fluctuations in the force. Hydraulics can generate a much higher force in a smaller size, deliver a constant force, and are less hazardous due to the compact nature with less moving parts.
II. Barrel: The barrel of a hydraulic cylinder is its outermost portion. The cylinder contains the other components that make up the hydraulic cylinder. This component is often painted or covered around its outside diameter as it is exposed to the surrounding conditions.
a. Function – The barrel must house all the components while operating at working pressures. Very close tolerances on the internal diameter must be held to prevent leaking oil as well as to reduce friction between the piston. The hollow tube must endure high pressures and is designed accordingly to prevent damage to the system or pose as a safety hazard.
b. Material – Barrels must be very strong to endure the operating pressures of the oil. Specialty steels are used to achieve this strength. Depending on the operating environment, the material can vary to achieve a barrel that is protected from water, high amounts of heat, or frigid regions. Engineering steels are common, such as C45E carbon steel. This material is a general-purpose steel used for many hydraulic barrels.
Material Contents – C45E is a non-alloy. Its composition is .42-.5% carbon and .5-.8% manganese. The carbon present increases the hardness of the steel. The manganese increases the hardenability of the steel while also lowering the hardening temperature.
Strength – C45E has a tensile strength of 600 MPa. The tensile strength of a material is represented by the load present per area of the specimen at failure. This method of loading is done by pulling axially at the ends of the material putting it in tension. Its strength makes it capable of enduring the stress present with hydraulics. The strength is once the material is in its finished state after being and hardened.
Hardening – This steel has a low hardenability in both oil and water. A hardened steel is essential in obtaining the mechanical properties necessary for hydraulics. Instead, the steel is surface hardened. The inside of the material remains unhardened while the outer surface is hardened. This leaves a strong and hard outside surface while the inside is left more flexible.
Potential material alternatives – While using a steel barrel is necessary to safely operate hydraulics at high pressures, various alloys can be used. Because the barrel is the outermost portion of a hydraulic cylinder, it is often made to resist any negative aspects in its surroundings. Environments exposed to water will find benefit from using a stainless steel. The added chromium as an alloying element will resist oxidization, however it will adversely impact the steels mechanical properties. Chromium will readily bond with carbon if it is not in solution to create chromium carbide, an undesired molecule that ruins the steels ability to be machined due to its hard nature. Care must be taken to ensure large amounts of carbon and chromium do not exist together. To do this, the amount of carbon is reduced in the steel which weakens it. Doing this causes the chromium to remain in solution. Coated bronze or aluminum may also be used to prevent oxidization from water, however they are far weaker than steel and are used for hydraulic systems that do not generate large amounts of pressure.
c. Method of Manufacturing – The manufacturing process of a hydraulic barrel beings with the material. Because it is a specialty non-alloyed steel, the material must be highly controlled in its composition. These quality steels are typically cold formed, so scale is not present. Stock is usually purchased as seamless tube to prevent large amounts of material waste. The long lengths of stock are shortened using an automatic band saw. The continuous blade will cut sections of the material and feed more stock making the process automatic. Another method available is by using a bar feeder. This will feed stock into the spindle of the machine once the previous part is finished. The cold rolled steel cylinders are then machined on a lathe. With modern machining capabilities, a CNC lathe is used. A CNC lathe is the most efficient as it runs programs that are easily altered or stored for future use. This method allows the rapid production of standard sized barrels. The cylindrical stock is loaded into the CNC machine. The end of the round stock is faced off to create a clean surface perpendicular to its axis while the steel rapidly rotates. The outer diameter is straight turned to the specified dimension. A drill is then inserted to remove stock. To obtain a more accurate hole, a boring head is used. This single point cutting tool slowly takes away material to correct the size and position of the bore. For even closer tolerances, a reamer can be inserted after boring. At both ends, the barrel is threaded. The threads will match with threaded caps that seal the cylinder. The part is then cut off and then precision ground. Based on the quality of the hydraulic cylinder, the tolerances will vary. A common method of achieving very close tolerances is honing. With honing, an abrasive stone is inserted into the bore while the part rotates. The high grit abrasive polishes the internal diameter making for a more effective hydraulic barrel. Barrels may also be casted. The dies required for the process are very costly, while they are permanent, they will wear and require replacement. Using permanent molds, die halves are closed together while liquid metal is injected into the cavity under high pressures. The barrel will cool and once solidified, the pressure is relieved, and the part is ejected, and the process can then be repeated. This process is rapid, but still leaves a finish that is not adequate and must be precision ground. Once the barrels are ground to tolerance, they are commonly hardened then painted. Because C45E does not have good hardenability in oil and water, the barrels are surface hardened. The parts are placed in a hot carbon rich environment. The carbon will diffuse into the very outer surface causing it to become harder. This process is known as carburization. After, many barrels will be painted to protect oxidizing. Care is taken to only cover the outer surface as the internal tolerances must be kept clean to ensure the very tight tolerances are kept.
Custom vs Continuous Manufacturing – When selecting a hydraulic cylinder, a major consideration is the bore size, or the internal diameter of the barrel. When a standard cylinder does not fit specification, a custom barrel can be produced. The process of manufacturing remains the same, however more flexibility is available such as material choice and size. By doing so, a CNC program specific to that barrel must be written. This results in a pricier part, however may be necessary for the situation. Continuously manufactured barrels come in a number of standard sizes. These can be mass produced by ordering stock in bulk and reusing already existing CNC programs. This overall results in a cheaper product. Custom barrels are typically not casted as the price of a custom die far exceeds the cost of machining.
III. Rod: The rod of a hydraulic cylinder is what transmits the force into an action. The pressurized fluid causes the rod to extend or retract. The rod is found inside the barrel and can be seen during the extension portion of the motion and is hidden within during its retraction.
a. Function – The rod of a hydraulic cylinder is what the load will be attached to. This cylindrical rod is subjected to forces outside of the hydraulic cylinder. Care must be taken to properly design and implement the correct rod, or it will fail under too high of a force and pose a serious safety hazard.
b. Material – Rods must be very hard and strong to withstand the forces exerted upon them. Steel is typically used with a hard coating to extend the life span of a cylinder.
Material Contents – The steel chosen is can vary, however it is typically a medium carbon content steel, such as 1040. This steel has .4% carbon and is both strong and tough. To increase surface hardness, the rods are coated with chromium. This coating also helps prevents corrosion. Stainless steel rods may also be used and coated with chromium when more resistance to the environment are required such as underwater use. Another coating, nitride, is used for added durability and wear resistance. This coating appears black while the chromium finish appears silver.
Strength – 1040 steel has a tensile strength of 620 MPa. Unlike the steel used for the barrel, 1040 reacts well to hardening and is quenched and tempered to increase its mechanical properties. The rod must be capable of withstand strong tension and compression forces. Another factor to consider is side loading. Hydraulics work axially, if a load is applied off axis, the rod risks bending. Any slight deviation in the path of the rod will ruin the cylinder as the tight mating tolerances must be kept.
Hardness – The chrome or nitrite plated rods gets their hardness from induction hardening. This process uses and electromagnetic field to excite the particles within the steel to rapidly heat them. They are then quenched to harden the metal. Induction hardening can be localized, making the process very efficient if areas of the rod are to be further machined therefore unhardened.
c. Method of Manufacturing – Rods are simpler to manufacture. From the cylindrical stock, the geometry does not change, only the size and finish. The rods are cut to length with an automatic band saw or a bar feeder through the spindle of a lathe. The rods are straight turned on a CNC lathe. Little machining is required on the lathe. They may be threaded on both ends to allow a connection to be mounted to one end. An alternative option would be through welding. One end is almost always threaded to allow the connection of a piston. The rods are parted off. They are then induction hardened. A copper coil subjected to large amounts of current will induce a magnetic field around the steel and heat it to quenching temperature. The quenched steel is much harder as a result but is brittle. The rods are tempered to relieve the steel of its brittle character. Lastly, the rods are precision ground. The rods undergo a process known as centerless grinding. The work piece lies on a work rest. A regulating wheel feeds the part into an abrasive grinding wheel. Higher end rods can be superfinished according to an even tighter tolerance. Superfinishing is used mostly on external cylindrical surfaces, like the rod of a hydraulic cylinder. The part is rotated while an abrasive stone oscillates polishing it. Tighter tolerances on the rod help to provide a better seal so less debris enters the cylinder and to prevent excessive wear on the seals between the rod and barrel.
Custom vs Continuous Manufacturing – Rods are continuously manufactured in a means similar to the barrels. CNC program codes can be reused to rapidly produce many stock sized rods. If a custom rod is required, a specific program must be written. Custom rods may be shorter or larger in diameter than a stock size to resist against side loading. The price of a custom piece is higher than that of a stock rod as the process cannot be scaled up to be more efficient.
IV. Piston: The piston of a hydraulic cylinder is attached to the rod or sometimes referred to as the piston rod. The piston is a cylindrical piece that is housed inside the cylinder. This component is responsible for converting hydraulic oil into movement. The oil pushes against the surface of the piston causing it to extend or retract.
a. Function – The piston sits entirely inside the hydraulic barrel. The piston responds to changes in hydraulic pressure. High pressures result in a high force transferred to the rod. The piston very closely matches the internal bore size of the barrel to prevent the leaking of oil which would reduce the cylinder’s ability to perform as efficiently.
b. Material – The piston is typically steel. A common grade is 1215 alloy steel. This steel provides ease of machining as well as adequate strength and durability.
Material Contents – 1215 alloy steel contains .09% carbon, .75-1.05% manganese and other trace elements. The steel is low in carbon and therefore predominantly iron. The ductile material combined with the presence of graphite causes the material to be more wear resistant.
Ease of Machining – 1215 steel is not very hard due to its low carbon content. This makes the material easy to machine. Larger depths of cuts and increased speeds and feeds increase efficiency of production.
Wear Resistance – The resistance to wear of the piston is attributed to the graphite present. Graphite is a softly bonded form of carbon, what is seen in pencil lead. Because of its soft characteristic, the molecule will self-lubricate against the inside of the barrel to extend the life of the piston.
c. Method of Manufacturing – The piston, like the other components, begins as stock. The steel rods must be shorted by means of an automatic band saw or bar feeder on a lathe. Because of the low carbon content, the pistons can be rapidly machined. The parts are straight turned on a CNC lathe. The pistons are then threaded to allow attachment to the piston rods. Afterwards, the pistons are parted off. Unlike the barrels and rods, the pistons are not hardened. The effectively quench steel, carbon must be abundant. 1215 steel does not have the carbon necessary, so it does not get the advantage of having its mechanical properties increased through hardening. Precision grinding is required to ensure a close fit between the piston and barrel. Like the piston rod, the piston is centerless ground. This achieves a fine surface finish. The pistons may also be superfinished to obtain an even finer finish. A tight tolerance is required for pistons to prevent wear and help ensure an effective seal.
Custom vs Continuous Manufacturing – Pistons can be rapidly produced in stock sizes for the majority of applications. However, if a custom piston is required it can be custom produced in the same manner as seen with the hydraulic barrels and piston rods. Custom pistons are fabricated in the same fashion, however specific setups and CNC programs are required that raise the price compared to those made through continuous manufacturing.
V. Conclusion – The components of a hydraulic cylinder list far longer than just the barrel, rod, and piston, however these three combine to make in essence the functioning part of the hydraulic cylinder. The material selection and method of manufacture has been greatly improved upon and continues to do so to achieve more efficient hydraulic solutions. Through polished surface finishes, hydraulics can reliably exert high forces and pressures for almost any size of job through either continuous or custom manufacturing processes.
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