Machinery's Handbook, 31st Edition
FLUID POWER ACTUATORS 2791 Kinetic Energy and Linear Actuators: Kinetic energy is generated when a load is moved. When a moving load is stopped, the kinetic energy is absorbed by the mechanism that stops it. Actuators can have either internal or external stops. External stops can gener- ally withstand greater impacts than internal stops. Manufacturers normally publish the kinetic energy limits of their actuators. Safe Loading of Linear Actuators: Rod failure and piston wear is of concern when lin- ear actuators are used in compression or are subjected to side loading. Side loading can be a result of external forces, eccentrically mounted loads, or mounting and operating a cylinder at some angle rather than vertical. It is critical to align connections to a cylinder such that side loading due to misalignment or eccentric loading is minimized. Compliant connections are often used to eliminate overcontraint and resultant loading. When rod bending or buckling is a risk, the use of external bearings or double rod cylinders can help reduce the risk. For horizontal cylinder installations, center trunnion mounting can help reduce the side loading on the cylinder. Another way to reduce premature failure due to side loading is to limit the extension stroke of a cylinder so that it cannot fully extend. The potential for buckling must be evaluated for any cylinder in compression. To in- crease the buckling strength of a cylinder, increase the piston rod diameter. Critical buck- ling load is calculated as shown in the following equation. Equivalent length is used to calculate critical buckling load rather than actual rod length. The equivalent length is dependent on the end conditions of the cylinder. The length L used to calculate equivalent length is assumed to be the distance at maximum extension between the exposed end of the piston rod and the nearest point of fixation. F cr π 2 EI L e 2 = ------ I π r 2 4 = ---- where F cr is the critical buckling load, E is the modulus of elasticity for the rod material, and L e is given in Table 29. Table 29. Equivalent Length, Linear Actuator Actuator and Load Fixations Equivalent Length L e Actuator Rigidly Mounted, Load Free 2 L Actuator and Load Pin Connected L Actuator Rigidly Mounted, Load Pinned 0.7 L Actuator and Load Rigidly Mounted 0.5 L Intensifiers and Hydraulic Presses.— Intensifiers use the energy in a relatively large vol - ume (work cylinder) of moderate pressure fluid to displace a smaller piston (ram). The two pistons are usually mechanically linked, so their stroke is the same. Intensifiers will hold a fixed pressure without additional input energy. The intensifier ratio, output pressure, and stroke are found using the following equations. R I p o p i = --- p o p i A i A o = ----- S 1 V w V C + A r ---------- l + =
p r 1000 ------
V o V C + A r ---------- X l + + =
=
0.1 V oil
S 2
V C
where R I = the intensifier ratio p o and p i = output and input pressures A o and A i = the two different piston areas
S 1 = the stroke of a single pressure intensifier V w = the volume of the work cylinder
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