Isolating the Vibrations of Forging Hammers
During the industrial revolution, methods of forging metal into desired shapes increased in scope and speed from their blacksmithing origins to larger industrial plants with steam-, pneumatic- and hydraulic-powered hammers. As the size of parts and forging speeds increased, so too did the vibration of the process. Early attempts at reducing shock involved timbers, leather, un-vulcanized rubber, loose soils and sometimes combinations of these materials.
Modern forging hammers are more massive and powerful than early designs. Consequently, hammer builders and users recognize that vibration isolation systems are desirable because of the improved work environment and to minimize the effect of vibration on surrounding equipment such as furnaces and machine tools. This article will address the technical issues involved in the isolation of forging hammers.
Forging Hammer IsolationTwo main designs are used in modern isolation systems: coil springs and elastomer-based systems. Coil springs with added dampers provide the greatest isolation performance but have higher initial cost and added maintenance concerns. Friction and viscous-type dampers added to spring isolators are difficult to protect from pit flooding, scale and other debris. Coil springs are also susceptible to damage from debris and fatigue from many forging blows.
Elastomer or vulcanized rubber-based systems are typically stiffer and thus somewhat less effective in isolating hammer shock. Most modern elastomers are resistant to oil and water contamination. They also have hysteresis damping in which the material dissipates system motion by heat. Elastomer systems offer superior isolation performance over timber and thin pad systems without the larger inertia mass, flooding issues and higher maintenance associated with coil-spring isolators. They are easy to install and maintain, and they are durable.
Analysis of the isolated hammer system can be best understood by separating the forging blow into three segments: when the ram is falling, when the ram is performing work and when the ram rebounds.
When the Ram is Falling
Hammer capacity is rated by the energy that can be delivered by the falling mass, which includes the ram and upper die. Most hammers are designed such that the falling weight impacts the workpiece at 6-7 meters/second (18-23 feet/second). The energy capacity is found by the following equation:
E = 1/2 • w/g • Vi2
The falling mass is calculated by taking the falling weight, w, and dividing by one gravity, g (9.8 m/s2 or 32.2 ft/s2). The impact velocity, Vi, should be in units of m/s or ft/s. The units of energy capacity, E, are Joules for metric and foot-pound force for imperial measure.
Hammers that operate by picking up the falling mass and releasing it with gravity providing all the acceleration are called drop hammers. Their energy capacity may be determined by multiplying the falling weight by the height of the drop, h, as follows:
E = wh
Hammers that accelerate the falling weight by using a piston powered by steam, hydraulic or pneumatic pressure hit with higher blow rates. In such systems, sufficient damping must be applied so that there is little or no movement when the next blow occurs. If the system is traveling downward when the next blow arrives, the blow will increase the amplitude of the downward motion more than the prior hit, possibly overstressing the isolation system. If the falling weight is accelerated quickly by the piston, the recoil may unload the isolation system, possibly leading to instability. Knowing the impact velocity and the falling weight is critical to designing a proper isolation system.
The short time in which the ram contacts and deforms the workpiece is the most important of the hammer’s operation. In practice, the magnitude of the blow and its duration can vary significantly. Hot open-die blows will generally impart lower magnitude and longer force duration between the ram/part/anvil than a finishing blow. Hammer builders understand that to develop maximum force on the part, the anvil must be much more massive than the ram. Figure 2 shows the theoretical hammer force relative to an extremely massive anvil that is 100 times as large as the ram.
The use of soft isolation systems will slightly decrease the peak force of the blow, an effect of great concern to hammer users. However, this effect is overstated. Because the anvil is much more massive than the ram and the blow force is of short duration, the force reduction is very small, usually less than 0.02%.
Hammer support systems using oak timbers may serve as a benchmark to compare other isolation systems. The time for one oscillation of the anvil is the natural period of the hammer system. Even with timber support, the hammer’s natural period is usually greater than the time the ram is in contact with the work. This is known as the shock impulse duration. The vibration transferred to the foundation, which would normally be of high magnitude and short duration on a timber system, is reduced to several lower-magnitude, lower-duration oscillations with an isolation system.
The transmitted shock of the hammer is reduced if the system’s natural period is at least six times greater than the shock force duration. The profile of the force imparted to the anvil during workpiece deformation does not make much difference in the force transmitted or in the motion response of the anvil if a soft isolation system is used. The time duration for various blows varies with the material, job and hammer.
The collision of the ram and workpiece transfers the momentum of the ram into downward motion of the anvil and the upward rebound of the ram. Once the ram and anvil reach the same velocity, the ram is finished doing work on the part. After this point in time, the ram rebounds upward and the anvil continues to travel downward.
Once the work has been done on the workpiece and the ram is rebounding, the impact from the ram is transferred to the anvil, and the isolation system controls the motion and transmitted forces. Because the shock impulse is of short duration, the hammer system can be accurately modeled using the conservation of momentum principle. Although some energy is transferred to the workpiece on impact, the conservation of momentum laws still apply.
m1vi + m2v2i = m1vf+m2v2f
Where: m1 = ram mass; m2 = anvil mass; vi = ram velocity immediately before impact; v2i = anvil velocity immediately before impact; vf = ram velocity immediately after impact; and v2f = anvil velocity immediately after impact.
The ram will not rebound at the same velocity as it fell; this change can be captured in the Coefficient of Restitution, CR, defined as:
CR = v2f – vf
vi – v2i
Open-die forging operations that cause very large deformations in a hot workpiece will have low CR values. As the workpiece cools with little deformation taking place, as in the case of finishing blows in a closed-die forging, CR values are higher.
If the foundation is large and bears on decent soil or piles sunk to bedrock, the foundation may be considered rigid, and the system has only one degree of freedom in the vertical direction. This single degree of freedom system is idealized as a simple spring and dashpot as shown in Figure 4.
The dynamic stiffness, K, of the isolation system determines the amount of motion and force transferred to the foundation. The damping component, x, of the system dissipates energy as heat.
After the ram has struck the workpiece and its momentum is transferred to the anvil, the anvil will oscillate about the equilibrium position upon the isolation system at what is called the system’s damped natural frequency, given by:
Ωd√ K/m2 • 1 - x2
With modern elastomer compounds, vertical natural frequencies of 8 Hz (Hertz=cycles/sec) are achievable. Coil springs can achieve natural frequencies of 5-7 Hz in directly supported hammers without adding inertia mass. Coil-spring systems with significant inertia masses have been installed with natural frequencies of 2-3 Hz for the best isolation currently achievable. Typical isolation efficiency is 60-80% reduction compared to traditional oak-timber systems.
In ConclusionThe motion of a hammer system is reduced when the anvil weight or natural frequency is increased. If a generally accepted limit of about 0.25-inch (7-mm) peak motion is applied, then for coil-spring and elastomeric systems there may be a need for the anvil to weigh more in order to prevent excessive motion, as shown in Figure 5.
By Hooke’s Law, the force transmitted to the foundation is the product of the isolation-system dynamic stiffness and the anvil motion. Because the motion of an elastomeric-based system is significantly less than a coil-spring isolator system, damping can be less and still maintain a stable system.
Author Steve Veroeven PE, is vice president of engineering for Vibro/Dynamics Corporation, Broadview, Ill. He may be reached at 800-842-7668 or firstname.lastname@example.org. Vibro/Dynamics Corporation has more than 40 years experience installing isolation mounts on stamping and forging presses and over eight years experience installing hammers. Vibro/Dynamics welcomes feedback from hammer users and OEMs on this article. For additional information visit www.vibrodynamics.com.