Full Ceramic Bearings in the 10,000-Year Clock. A Jeff Bezos-Led Project
The 10,000-Year Clock is a mechanical timekeeping system designed to run continuously for the next ten thousand years under its own power. The Clock displays the positions of the stars and planets as well as the date.
In addition, the Clock sounds a unique melody with its ten bells it will do this every day for the next ten thousand years through a massive analog computer, so long as humans wind the chime mechanism.
The Chime Generator is a mechanical computer that uses gears and sliding pins to calculate a new sequence of notes each day. The Clock will maintain precision to within one day in 20,000 years. This is a formidable task considering the eventual lengthening of the day due to changing tidal forces and inevitable minute construction errors. The Clock uses a solar synchronization system, which calibrates the more traditional mechanical time-keeping machinery with the noonday sun to reconcile solar and sidereal time. In the event of a nuclear winter or natural volcanic event blocking out the sun for decades, the clock will be able to recalibrate itself once the sun re-emerges. To ensure its lasting functionality, the clock has been designed to run for many years without any input energy from human winding or energy from the day-night thermal cycle. Given the intended design life of the Clock, this is perhaps the farthest forward-reaching engineering project currently in progress.
Nearly every component of this complex system depends on Boca Ceramic Bearings. From the large pinion gears and winding differentials of the Main Power System to the bell trigger mechanisms of the Chime Generator, these dependable, highly durable bearings are at the heart of the Clock.
Work on The 10,000-Year Clock project began in 1996 with the creation of the Long Now Foundation, a nonprofit organization fostering long-term thinking and responsibility on a 10,000-year timescale. An eight-foot functioning prototype of the clock was finished on New Year’s Eve 1999.
The clock’s location inside a mountain, the materials it is constructed from, and the design conventions applied to its mechanics minimize the effects of changing temperature and humidity. The clock’s designers have been extremely selective when it comes to 10,000-year-approved materials. Even with the most corrosion-resistant metals, the clock’s gears and linkages are designed to keep working in the worst-case projections for corrosion.
Key factors in clock material selection:
The Clock is primarily constructed from three materials: 316 stainless steel, titanium, and certain ceramics. Nearly all the ceramic components are dry-running Boca Zirconia Oxide Full Ceramic Radial Bearings. With their high resistance to wear and corrosion, these bearings provide consistent long-term performance. Zirconia bearings are hard enough that dirt and small pieces of metal debris from wear on the clocks gear systems have not caused a reduction in performance over time in cycle testing. As a result, nearly all rotating components in the 10,000-year Clock utilize Boca Ceramic Bearings.
Testing
Representative components of the clock have been cycled and tested to ensure they will survive the cycles we plan to put them through in their 10,000 working life. To do so, we built various cycle-testing setups to simulate the cyclic loads that gears, springs, and bearings will experience over the lifetime of the Clock. In design and prototyping for the Clock, engineers at the Long Now Foundation tested a range of metal, hybrid, and full ceramic bearing options. Ceramic bearings, specifically Yttria-stabilized zirconia, is an obvious choice for long-term applications because of their resistance to corrosion and durability. In our extensive cycle testing, only full ceramic bearings perform within tolerance for the working life and accuracy required by the Clock. Ceramic bearings are also the only option that does not require added lubrication, a must for a system that may run unattended for decades or even centuries beyond the working life of currently available lubricants. Some bearings in the Clock have been tested with positive results through more than 350 million cycles. They have consistently exceeded our requirements and expectations.
Below we will cover bearing applications in the main drive system of the 10,000-Year Clock. Some components will only rotate on their bearings once or twice in the entire life of the Clock, while others will experience millions of cycles under loads approaching design strength. We need bearings that can accommodate this wide range of applications with a comfortable safety factor.
We chose Boca Ceramic Bearings for the following traits:
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Examples
We will look at three examples of ceramic bearing applications, including a high-load scenario and a somewhat unusual use of ceramic bearings as contact surfaces in pinion gears.
Cycloid Bevel Drive
Throughout the Clock’s drive system torque is geared down to different components of the mechanism. One way this is achieved is through cycloidal bevel gears. Cycloidal gears are used in mechanical clocks to ensure that the motion of one gear is transferred to the next with a locally constant angular velocity. We use cycloidal gears instead of involute gears because they are more efficient. They allow for the use of rollers and rolling friction is several orders of magnitude lower than the sliding friction experienced by an involute gear pair. As shown in the diagram below (note how one gear axis is vertical but its mating gear’s axis is horizontal), both the rollers and the gear teeth are beveled to translate motion from one plane to another 90 degrees from the first. In this gear system, ceramic bearings are used to reduce resistance in the cycloid bevel pinion (the left-hand assembly in Fig. 2) by allowing the beveled rollers themselves to spin freely as they mate with the bevel gear. For the life of the Clock, these bearings will be under constantly changing loads as the pinion spins and different rollers come into and out of mesh with the mating gear. Fortunately, in our tests to failure, the gears themselves typically wear to the point of dysfunction before the bearings show wear that affects our tolerances.
Pinion Drive
Visitors to the Clock can add energy to the Clock’s storage system by winding up a large mass. As they wind a capstan, a series of differentials turn three large pinion gears. These gears mesh with the three arms of the rack segments, which in turn connect the power system to the counterweight. The slow descent of the mass is the primary driver of the Clock. To reduce resistance in the system, the pins in each pinion-gears are free to move on ceramic bearings. These bearings experience some of the highest loads in the clock at over 12,000 N of constant lateral force. After putting prototypes through millions of cycles at such forces, the system was redesigned to house larger bearings to avoid pitting on the surface of the zirconia balls. To further increase the working life of the system, ceramic sleeves house the stainless steel pins of the pinion gear and are the surface that contacts the arms of the rack segments.
Cycloid Drive
In this final example, we see a common gear arrangement with a novel application of ceramic bearings. In this cycloidal drive, a 316 stainless gear meshes directly with 608 zirconia bearings. The part count is reduced by using bearings as the contact surface for the right-hand gear. Instead of using a stainless pin on two bearings a single bearing does the job. Because much of the clock's mechanical systems are made from relatively soft 316 stainless, galling is a major concern. To avoid galling and reduce the coefficient of friction most of our gear pairs have 316 stainless gear mating with a titanium gear. In this case, we achieve the same with the zirconia oxide bearings' outer race meshing with the stainless teeth.
See the final clock installation in this article: Bearing Company Helps Bezos with 10000-Year Time Project.
Watch the video here: Boca Bearing Spotlight: 10,000-Year Clock
Contact Boca Bearings at info@bocabearings.com
Tel: 561-998-0004
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