Come friends today we will know about Hydraulic Actuator. A hydraulic cylinder (also called a linear hydraulic motor ) is a mechanical actuator used to deliver a unidirectional force through a unidirectional stroke . It has many applications, particularly in construction equipment ( engineering vehicles ), construction machinery and civil engineering.
Surgery
Hydraulic cylinders get their power from pressurized hydraulic fluid , which is usually oil . The Hydraulic Actuator of a cylinder barrel , in which a piston connected to a piston rod moves back and forth. The barrel is closed at one end by the bottom of the cylinder (also called the cap) and the other end by the cylinder head (also called the gland) where the piston rod comes out of the cylinder. The piston consists of sliding rings and seals. The piston divides inside the cylinder into two chambers, the lower chamber (cap end) and the piston rod side chamber (rod end/head-end).
Flanges , trunnions , clevises , and lugs are common cylinder mounting options. The piston rod also has mounting attachments to connect the cylinder to the object or machine component that it is pushing or pulling.
A hydraulic cylinder is the actuator or “motor” side of this system. The “generator” side of the hydraulic system is the hydraulic pump that delivers a fixed or regulated flow of oil to the Hydraulic Actuator to move the pistons. Three types of pumps are widely used: hydraulic hand pump, hydraulic air pump and hydraulic electric pump. [1] The piston pushes the oil back into the reservoir in the second chamber. If we assume that during the expansion stroke, oil enters from the cap end, and the oil pressure at the rod end/head end is almost zero, then the force F on the piston rod is the pressure in the cylinder times the piston area A is equal to P. ,
F=P\cdot A
Retraction force difference
For a double-acting single-rod cylinder, when the input and output pressures are reversed, there is a force difference between the two sides of the piston because one side of the piston is covered by the connected rod. The cylinder rod reduces the surface area of the piston and reduces the force that can be applied to the retraction stroke. [2]
During the retraction stroke, if oil is pumped into the head (or gland) at the rod end and the oil from the cap end flows back into the reservoir without pressure, the fluid pressure in the rod end is (pull force ) / ( piston area – piston rod area):
P={\frac {F_{p}}{A_{p}-A_{r}}}where p is the fluid pressure, f p is the pulling force, a p is the piston face area and a r is the rod cross section area.
For a double-acting, double-rod cylinder, when the piston surface area is evenly covered by rods of the same size on both sides of the head, there is no force difference. Such cylinders usually have their cylinder body affixed to a fixed mount.
Application
Hydraulic Actuator are used in earth-moving equipment to raise or lower a boom, arm, or bucket . These cylinders are also used in hydraulic bending machines, metal sheet shearing machines, hot press forming for making particle board or plywood .
Parts
The hydraulic cylinder consists of the following parts:
Cylinder barrel
The main function of the cylinder body is to control the cylinder pressure. The cylinder barrel is mostly made of honed tubes. [3] Honed tubes are manufactured from suitable two-honed steel cold drawn seamless tubes (CDS tubes) or draw over mandrel (DOM) tubes. Honor tubing for hydraulic cylinders is ready for use without further ID processing. The surface finish of a cylinder barrel is typically 4 to 16 microinches. The honing process and the skiving and roller burning (SRB) process are the two main types of processes for manufacturing cylinder tubes. [4] The piston rotates in the cylinder. The cylinder barrel has the characteristics of smooth inner surface, high precision tolerance, durable in use, etc.
Cylinder base or cap
The main function of the cap is to enclose the pressure chamber at one end. The cap is attached to the body via welding, threading, bolts or tie rods. Caps also act as cylinder mounting components [cap flange, cap trunnion, cap clevis]. Capsize is determined on the basis of bending stress. A static seal/o-ring is used between the cap and barrel (except for welded construction) .
Cylinder head
The main function of the head is to surround the pressure chamber from the other end. The head has the option of accepting an integrated rod sealing arrangement or a seal gland. The head is attached to the body via threading, bolts or tie rods. A fixed seal/o-ring is used between the head and the barrel .
Piston
The main function of the piston is to isolate the pressure zones inside the barrel. Pistons are machined with grooves to fit elastomeric or metal seals and bearing elements. These seals can be single-acting or double-acting. The expansion and retraction of the cylinder occurs due to the difference in pressure between the two sides of the piston. The piston is attached to the piston rod by means of threads, bolts or nuts to transfer linear motion.
Piston rod
The piston rod is typically a hardened chrome-plated piece of cold-rolled steel that attaches to the piston and extends from the cylinder through the rod-end head. In a double rod-end cylinder, the actuator has a rod exiting either side of the piston and from both ends of the barrel. The piston rod connects the hydraulic actuator to the working machine component. This connection can be in the form of a machine thread or a mounting attachment. The piston rod is highly ground and polished to provide a reliable seal and prevent leakage.
Seal gland
The cylinder head is fitted with a seal to prevent pressurized oil from leaking out from the interface between the rod and the head. This area is called the seal gland. The advantage of the seal gland is easy removal and seal replacement. The seal gland consists of a primary seal, a secondary seal/buffer seal, bearing elements, a wiper/scraper, and a static seal. In some cases, especially in small Hydraulic Actuator, the rod gland and the bearing element are composed of a single integral machined part.
Seals
The seal is considered according to cylinder working pressure, cylinder speed, operating temperature , working medium and application. Piston seals are dynamic seals, and they can be single-acting or double-acting. Generally speaking, elastomer seals made of nitrile rubber , polyurethane, or other materials perform best in low-temperature environments, while seals made from fluorocarbon vitons are better for higher temperatures. Metallic seals are also available and cast iron is commonly used for the seal material. Rod seals are dynamic seals and are generally single-acting. Rod seal compounds are nitrile rubber , polyurethane or fluorocarbon viton, Wipers/scrapers are used to eliminate contaminants such as moisture, dirt and dust, which can cause extensive damage to cylinder walls, rods, seals and other components. The common compound for wipers is polyurethane. Metal scrapers are used for sub-zero temperature applications and applications where foreign material may accumulate on the rod. Bearing elements/wear bands are used to eliminate metal to metal contact. The wear bands are designed according to the side load requirements. The primary compounds used for the wear bands are PTFE , woven fabric reinforced polyester resin, and bronze filled
Other part
There are several component parts that make up the interior of a hydraulic cylinder. All of these pieces combine to form a fully functioning component.
- cylinder base connection
- cushion
- Internal Threaded Ductile Heads
- head glands
- Polypack Piston P
- cylinder head caps
- butt plates
- Eye Brackets / Clevis Brackets
- mp3 detachable mounts
- Rod Eyes / Rod Clevis
- pivot pin
- Round ball bushings
- circular rod eye
- alignment coupler
- ports and fittings
Single acting vs double acting
- Single-acting cylinders are the economical and simplest design. Hydraulic fluid enters through a port at one end of the cylinder, which spans the rod through the area gap. An external force, internal reciprocating spring or gravity returns the piston rod.
- Double acting cylinders have a port at each end or side of the piston, which is supplied with hydraulic fluid for both retraction and expansion.
Design
There are mainly two main styles of hydraulic cylinder manufacturing used in industry: tie rod-style cylinders and welded body style cylinders.
Tie rod cylinder
Tie rod style hydraulic cylinders use high strength threaded steel rods to hold the two end caps to the cylinder barrel. They are often seen in industrial factory applications. Small-bore cylinders typically have 4 tie rods, and large-bore cylinders may require as many as 16 or 20 tie rods to maintain the end cap under the tremendous forces produced. Tie rod style cylinders can be completely disassembled for service and repair, and they are not always customizable. [7]
The National Fluid Power Association (NFPA) has standardized the dimensions of hydraulic tie rod cylinders. This enables cylinders from different manufacturers to be interchanged within the same mounting.
Welded body cylinder
Welded body cylinders have no tie rods. The barrel is welded directly to the end cap. The ports are welded to the barrel. The front rod gland is usually threaded or bolted to the cylinder barrel. This allows the piston rod assembly and rod seal to be removed for service.
A cut away of a welded body hydraulic cylinder showing the internal components
Welded body cylinders have several advantages over tie rod-style cylinders. Welded cylinders have a narrower body and often a shorter overall length which enables them to fit better within the tight confines of machinery. Welded cylinders do not suffer from failure due to tie rod stretch at high pressures and long strokes. The welded design also lends itself to customization. Special features are easily added to the cylinder body including special ports, custom mounts, valve manifolds, and more. [7]
The smooth outer body of the welded cylinders also enables the design of multi-stage telescopic cylinders. Welded body hydraulic cylinders dominate the mobile hydraulic equipment market such as construction equipment ( excavators , bulldozers, and road graders) and material handling equipment (forklift trucks, telehandlers, and lift-gates). They are also used by heavy industry in cranes, oil rigs, and large off-road vehicles for above-ground mining operations.
Piston rod manufacturing
The piston rod of a hydraulic cylinder is driven both in and out of the barrel, and consequently both in and out of the hydraulic fluid and the surrounding environment.
Coatings
Wear and corrosion resistant surfaces are desirable on the outer diameter of the piston rod. Surfaces are often applied using coating techniques such as chrome (nickel) plating, Lunac 2+ duplex, laser cladding, PTA welding and thermal spraying. These coatings can be finished to a desirable surface roughness (Ra, Rz) where the seals give optimum performance. All of these coating methods have their own specific advantages and disadvantages. This is why coating specialists play a vital role in selecting the optimum surface treatment process to protect hydraulic cylinders.
The cylinders are used in a variety of operating conditions and this makes finding the right coating solution a challenge. In dredging may be impacted by stones or other parts, in saltwater environments, there are extreme corrosion attacks, off-shore cylinders have to withstand bending and impact in combination with saltwater, and in the steel industry, high temperature, etc. There is no single coating solution that successfully combats all typical operating wear conditions. Every technology has its advantages and disadvantages.
Length
Piston rods are generally available in lengths that are cut to suit the application. Since ordinary rods have a soft or mild steel core, their ends can be welded or machined for screw threads .
Distribution of forces on components
The forces on the piston face and piston head retainer differ depending on which piston head retention system is used.
If a circle (or any non-preloaded system) is used, the force acting to separate the piston head and the cylinder shaft shoulder is the pressure multiplied by the area of the piston head. The piston head and shaft shoulder will be separated and the piston head responds fully to the load by the retainer.
If a preloaded system is used the force between the cylinder shaft and the piston head is initially the piston head retainer preload value. This force will decrease once the pressure is applied. The piston head and cylinder shaft shoulder shall be in contact until the pressure exerted by the piston head area exceeds the preload.
The maximum force the piston head retainer will see is greater than the preload and the applied pressure multiplied by the full piston head area. The load on the piston head retainer is greater than the external load, which is caused by a reduction in the size of the shaft passing through the piston head. Increasing this part of the shaft reduces the load on the retainer.
Side loading
Side loading is uneven pressure that is not concentrated on the cylinder rod. This off-centre strain can cause bending of the rod in extreme cases, but usually causes leaks by rotating the circular seals into an oval shape. It can also damage and enlarge the bore hole around the rod and the inner cylinder wall around the piston head, if the seals are completely compressed and deformed to make metal-on-metal scraping contact with the rod. is pressed sufficiently. [9]
Side loading stresses can be directly reduced by the use of internal stop tubes that reduce the maximum extension length, leave some distance between the piston and the bore seal, and increase leverage to resist warping of the seals. Double pistons also dissipate the forces of side loading while reducing stroke length. Alternatively, external sliding guides and hinges can support the load and reduce side loading forces applied directly to the cylinder.
Cylinder Mounting Methods
Mounting methods also play an important role in cylinder performance. Generally, fixed mounts on the centerline of the cylinder are best to avoid straight-line force transfer and wear. Common types of mounting include:
Flange Mount – Very strong and rigid, but has little tolerance for misalignment. Experts recommend cap end mounts for thrust loads and rod end mounts where the dominant loading stresses the piston rod. There are three types head rectangular flange, head square flange or rectangular head. The flange mount works optimally when the mounting face is attached to the machine support member. [11 1]
Side-mounted cylinders – easy to install and service, but mounts generate a significant moment as the cylinder exerts force on the load, leading to wear and tear. To avoid this, specify strokes for side mount cylinders at least up to the bore size (heavy loading makes short strokes, large bore cylinders unstable). The side mounts need to be well aligned and the load supported and directed.
Centerline Lag Mounts- Absorb forces at the centerline, and require dowel pins to secure the lugs to prevent movement under high pressure or shock conditions. Dowel pins hold it to the machine when working under high pressure or shock loading. [11 1]
Pivot Mount – Absorb force on the cylinder centerline and allow the cylinder to change alignment in one plane. Common types include clevis, trunnion mounts and spherical bearings. Since these mounts allow a cylinder to pivot, they must be used with a rod-end attachment that also pivots. Clevis mounts can be used in any orientation and are generally recommended for short stroke and small to medium-bore cylinders. [12]
Special hydraulic cylinder
Telescopic cylinder
The hydraulic cylinder length is the total stroke, piston thickness, bottom and head thickness, and connection length. Often this length does not fit the machine. In that case the piston rod is also used as the piston barrel and another piston rod is used. Such cylinders are called telescopic cylinders. If we call a common rod cylinder single stage, then telescopic cylinders are multi-stage units of two, three, four, five or more stages. In general telescopic cylinders are much more expensive than normal cylinders. Most telescopic cylinders are single acting (push). Double acting telescopic cylinders must be specially designed and manufactured. [13]
Plunger cylinder
A hydraulic cylinder with a piston or without a seal is called a plunger cylinder. The plunger cylinder can only be used as a pushing cylinder; The maximum force is the piston rod area multiplied by the pressure. This means that a plunger cylinder normally has a relatively thick piston rod.
Differential cylinder
When pulling a differential cylinder acts like a normal cylinder. If the cylinder has to be pushed though, the oil is not returned to the reservoir from the piston rod side of the cylinder, but to the underside of the cylinder. In such a situation, the cylinder moves faster, but the more force the cylinder can give, it is like a plunger cylinder. A differential cylinder can be constructed like a normal cylinder, and only a special control is added.
The above differential cylinder is also called regenerative cylinder control circuit. This term means that the cylinder is a single rod, double-acting hydraulic cylinder. The control circuit includes a valve and piping that, during the expansion of the piston, moves the oil from the rod side of the piston to the other side of the piston rather than the pump’s reservoir. The oil that flows to the other side of the piston is called regenerative oil.
Position Sensing “Smart” Hydraulic Cylinder
Position sensing hydraulic cylinders eliminate the need for a hollow cylinder rod. Instead, an external sensing “bar” senses the position of the cylinder’s pistons using Hall Effect technology. This is accomplished by the placement of a permanent magnet within the piston. The magnet propagates a magnetic field through the steel wall of the cylinder, sending a detection signal to the sensor.
Glossary
In the United States, popular usage refers to the complete assembly of the cylinder, piston, and piston rod (or more) collectively referred to as “pistons”, which is incorrect. Instead, the piston is the small, cylindrical metal component that internally separates the two halves of the cylinder barrel.
