LITERATURE: Technical Articles

WHEN MACHINES COLLIDE

Simple safety couplings can prevent expensive damage that often results when cutting machines and other industrial equipment careen past their safety stops.

RAINER SCHNABEL
Jakob GmbH
GAM Enterprises, Inc.
7317 W. Wilson Avenue
Chicago, IL 60656
(708) 867-7860

From July 9, 1992, Machine Design. Reprinted with permission from Penton, Cleveland, Ohio.

Downtime for any production machinery is expensive. Collisions between cutting tool and end stops or workpiece fixtures produce the most costly repairs in metalworking. Also, machine tables become blocked while moving in feed or in rapid traverse. The collision forces on the table and spindle can get extremely high. Though these forces may last for only a few milliseconds, they still can cause expensive damages.

In a collision, the motor is a source of most inertial force. At impact, the motor puts out peak torque for the few milliseconds it takes the current overload relay to react. The relay will probably protect the motor from overheating. But the relay reacts too slowly to protect equipment from destructive collision forces. In fact, the momentary peak in torque produced by the motor acts as an additional shockload.

Because motor current relays are slow, mechanical devices provide better overload protection. Devices such as torque limiters cannot prevent collisions, but can do the next best thing by markedly limiting destructive inertial forces.

Enter safety couplings

Only a mechanical torque limiter can disconnect a motor from a spindle drive fast enough to head off damaging inertial forces. For common applications that include conveyor drives, business machines and various instruments, mechanical torque limiters such as shear-pin couplings or slip clutches may provide adequate protection. But they are impractical for high-performance uses typified by machine tools. One problem is that ordinary clutches and couplings introduce torsion and backlash into the axis being protected. There are other difficulties as well in typical applications.

Safety couplings, on the other hand, are a new type of mechanical torque limiter that overcome such conventional stumbling blocks. Safety couplings are optimized to secure high-speed, high-precision drive applications against overload damages. Rather than being a refinement on ordinary torque limiters, they have been conceived from scratch in cooperation with machine tool manufacturers to meet their particular needs and objectives.

The cost of protection with safety couplings is also low. It typically runs $750 per axis protected to equip a conventional machining center with the devices.

• Safety couplings all have some common features such as
• No backlash, even in the keyless shaft-hub connection;
• High torsional stiffness and very low moment of inertia; small size and infinitely adjustable disengagement torque;
• A retained reference point after re-engagement;
• A warning signal in case of an overload; and
• A choice of either self-acting re-engagement or no self-acting re-engagement. The devices are also temperature resistant for over 500°F.

In addition, certain types compensate for axial, lateral and angular shaft misalignment. Some also contain integral ball bearings for pulleys or chain wheels. Many of these features not only make safety couplings perfect for machine tool drives, but also for high-performance assembly lines, printing presses, packaging machines, and so forth.

Inside safety couplings

A safety coupling is basically a spring-loaded form-fit coupling that transmits torque through use of spring-loaded washers pressing balls into spherical indentations. The ball cage is in a thrust ring, the indentations in a shrouding ring. The thrust ring is fixed through a backlash-free connection to either a flexible metal bellow when used in direct drive applications, or to a pulley for indirect drives.

In the presence of an overload, the hub with its ball cage will overtwist and press the balls out of the indentations to disengage. Only the balls and the thrust washers move axially, resulting in a very low disengagement mass.

The acceleration of these low masses results in a minimal increase in dynamic torque. In contrast, acceleration of ordinary mechanical couplings, with their higher disengagement mass, produces a dynamic torque that exceeds static torque by a factor of two or three. Higher static torque, in turn, results in a higher destructive force.

Springs are key components in the operation of safety couplings. In conventional spring-loaded couplings, springs experience additional stress when activated.

Therefore, they create a certain amount of additional spring tension and torque until the couplings completely disengage. This tends to delay decoupling after a collision.

The disk springs developed for this application, however, do not operate this way. Disk springs function on the diminishing side of the spring characteristic curve (release their stored energy with little displacement), causing torque to drop off immediately upon activation. Thus, the coupling interrupts torque transmission at once.

The diminishing spring characteristic also eliminates problems that sometimes arise in conventional devices when operating torques are slightly below disengagement torque. Here, the coupling can sometimes slip or eventually fail.

A proximity switch can initiate an emergency stop or hold other machine operations immediately. Units can be designed to either automatically reengage after torque drops, or to remain disengaged until drive rotation has reversed. In the later case, an additional locking mechanism keeps the safety coupling disengaged. The machine reference point, however, is retained with either re-engagement mode.

Better connections

It can be difficult to connect shaft ends in direct drive applications. Even if the shaft alignments are close to perfect, it is only a matter of time before vibrations, heat, and wear knock them out of line. A rigid coupling would cause excessive bearing loads and affect the shaft velocity. To compensate for shaft misalignment, safety couplings for direct drive applications are therefore equipped with a metal bellow. The bellow is torsionally stiff but flexible in axial, angular and lateral directions. Torsional rigidity comes from the use of a patented press-fit method that joins the bellow and shaft hub. This method of attachment also provides considerably higher durability than welding or soldering.

This frictional shaft-to-hub connection eliminates costly keyways. It also permits transmitting more torque fro m smaller shafts.

Safety couplings come in versions with integral bellow s designed for direct drives (above) and mounted to pulleys or chain gears for indirect drives (left). The blue wire in the installation at left goes to a proximity sensor that detects when the coupling has disengaged and stopped turning.

Visible in this cross section of a direct drive safety coupling are the thrust ring, shrouding ring, and ball cage assembly that comprise the heart of the disengagement mechanism. Also visible is the optional locking mechanism that keeps the device disengaged until shaft rotation reverses. The proximity sensor detects the movement of the thrust ring and ball cage in the event of a collision. The sensor signal can then be used to signal a machine stop.

Graphs of typical collision forces on a workpiece carriage illustrate the benefits of safety couplings. The devices drastically cut the amount of force accidentally delivered to a carriage or a spindle.

Springs comprise a key component for safety couplings. Because they operate on the diminishing side of the characteristic curve, torque drops immediately upon actuation. Conventional torque limiters, in contrast, typically use springs operating on the increasing side of the curve. This causes the device to physically disengage at a torque level that exceeds the torque setting. Depending on the elements used to transmit torque, this action can, in the long run, destroy the coupling.