Floating armature revs up clutch/brake system

By: 
May 17, 1999

Early in the design of Pitney Bowes' 14 Series high-speed insertion machine, engineers realized in order to provide customers with higher throughput than ever before possible--14,000 documents an hour--they faced a real challenge. They would need a clutch/brake solution that could perform significantly better than conventional electromagnetic technology. Utilities, banks, and other companies use these machines to fold, insert, and apply postage to customer statements.

To put things into perspective, Pitney Bowes' new machine had to handle nearly four sheets of paper per sec. But after factoring in the time for a sheet of paper to physically move between three buffer stations within the machine, the clutch had a mere 25 ms to respond. In contrast, for a comparable, electromechanical conventional clutch, from the time voltage is applied to the coil to the time the driven shaft of the clutch reaches full speed is approximately 35 ms.

Worse yet, the fast cycle times meant components would wear out quicker. Engineers estimated that they would have to improve the life expectancy of the clutch/brake system by an order of magnitude.

In head-to-head testing, Ogura's zero-gap clutch/brake design outperformed the competition by reaching maximum speed faster.

Initially, servo technology appeared to be the only option that could meet these new performance levels for the customer. But as Technical Advisor John Sussmeier explains, engineers could not cost-justify it in this application. "The existing machine architecture, which we wanted to preserve for the 14 Series, was amenable to controlling motion through a simple, on/off controller. With servo, we would have needed a separate controller for each of the three motors in our machine, and the complexity involved would have made it cost-prohibitive."

Another alternative was to simply increase the voltage, but Sussmeier explains that this approach would have led to concerns about thermal management. "When you double the voltage, you quadruple the power, and then you have a heat problem. In other words, you have to be more conservative in your design so that you don't cook the coils," he says.

Fortunately, Sussmeier came across an electromechanical clutch/brake design from Ogura Industrial (Somerset, NJ) that he thought could meet the stringent requirements. And he compiled the test data to prove it out.

Zap the gap. A key aspect of the clutch/brake design, says Ogura's Dave Kane, is the absence of an air gap between the armature and the rotor. "Conventional designs have a space of about 0.25 mm, and since the strength of the magnetic field is a function of the inverse square of the distance, it obviously has an adverse impact on time-to-speed," he explains. "Also, since the armature rests against the rotor, we don't have to overcome any inertia to move that mass up against a friction surface."

Another drawback of conventional clutch/brake designs is a phenomenon known as the scrubbing effect, in which the ceramic friction material wears from repeated engage/disengage cycles. As the material wears away and the gap becomes larger, it takes longer for the coil to build up sufficient voltage to pull in the armature. Equally bad, cycle times become unpredictable until the gap widens to the point that the clutch can no longer engage.

Design engineers at Ogura overcame the time-to-speed issue in two ways: First, in order to increase torque, they used a low inductance/high efficiency coil that generates a magnetic field more quickly than standard coils. Second, they eliminated the air gap completely. Their ingenious, "zero-gap" design features a so-called floating armature, which sits on a splined hub that allows it to move axially.

Other gap-less designs employ a light spring to hold the armature in place. But Kane explains that the cycle times in this application are so fast that the clutch armature never has time to move away from the rotor. "We're taking advantage of the small amount of residual magnetism to hold the armature in light contact with the rotor at all time," says Kane.

He does add, however, that it was somewhat of a Catch-22. "At the same time we wanted to keep the armature in contact, we didn't want any drag on the clutch. The solution was to use materials that hold a little residual magnetism."

Solving one problem created another, as is typically the case with a new design. With the armature and rotor in constant contact, engineers needed a friction material that was hard enough to resist wear and meet the daunting life cycle requirements, but at the same time was not brittle or prone to flaking.

The solution was a ceramic composite material made exclusively for Ogura for use in clutch/brake applications. Although engineers would not divulge the exact composition of the material, they claim that its hardness is 2.5 to 3 times better than bearing or hardened steel. To minimize wear of the armature, they also used a nitrided chemical surface hardening treatment.

Another design innovation involved mounting the coil shell to sealed ball bearings, instead of bronze bushings. Because the bearings show no appreciable wear (unlike bushings), the armature and rotor surfaces hold truer contact--thereby decreasing any wear caused by concentricity differences.

So good were these design changes, in fact, that after conducting a head-to-head comparison of several brake designs, Pitney Bowes' Sussmeier was surprised with the test results. "We thought wear on the friction surfaces would be a problem, but in fact after 7 million cycles, they showed only about 0.10 mm of wear. With those kind of numbers, we estimate that the clutch/brake will last at least 25 million cycles--well within our requirements."

Anatomy of a zero-gap clutch/brake


1. clutch coil
2. clutch rotor
3. zero air gap
4. clutch armature
5. zero air gap
6. ceramic friction material
7. brake coil in coil shell
8. ceramic friction material
9. zero gap
10. brake armature
11. ceramic
friction material

Specs at-a-glance
Actuation: Electrical, 24V dc
Motion: Start/Stop random
Accuracy: Start: 10 ms from coil actuation
Stop: 10 ms from coil deactivation
Shaft diameter: 10 mm minimum
15 mm maximum
Speed: 1,000 rpm
Inertia: 0.5 lb-in2
Friction load torque: 3 in-lb
Cycle rate: 4/sec
Life expectancy: 25 million cycles
Note: Technical data appearing in this table relates to life-test results performed by Ogura. Some of the test specifications differ slightly from Pitney Bowes' actual application.


Underlying design principles

By eliminating the air gap between the armature and rotor in this clutch/brake design, engineers were able to increase the time-to-speed in two ways. Time-to-speed is the amount of time it takes after voltage is applied to the coil for the driven shaft of the clutch to reach full speed.

First, given that the strength of an electromagnetic field is proportional to the inverse of the distance to the armature, the flux density in the case of a zero gap is greater, resulting in greater torque. This relationship comes from the Biot-Savart Law, an inverse square law that is the magnetic equivalent of Coulomb's law.

According to the law, the magnetic field (dB) associated with a current element (dl) is given in magnitude by:

where:

i=
net current

r = displacement vector from the coil to the point at which we want to know the magnetic field

u = angle between this vector and dl

mu = magnetic moment of the dipole

Second, and more significantly, is the fact that the armature maintains light contact with the rotor at all times. Conventional clutch designs must overcome the inertia of the armature, which can be significant, and then pull it into the rotor.

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