Spring Drive

Spring Drive is a name given to a series of watch movements produced by Epson in Shiojiri. The concept of using a mainspring to power a quartz timing package was first conceived in 1977 by Yoshikazu Akahane (赤羽 好和) at Suwa Seikosha^[1] (now a part of Epson after a 1985 merger).^[2] Specified to one second accuracy per day,^[3] the movement uses a conventional gear train as in traditional mechanical watches, but rather than an escapement and balance wheel, instead features Seiko's Tri-synchro Regulator system in which power delivery to the watch hands is regulated based on a reference quartz signal.^[3]

Commercially released in 1999,^[2] the movement is found in watches distributed by the Seiko Watch Corporation, including its Credor, Grand Seiko, Presage, and Prospex brands.

The Spring Drive uses a conventional mainspring^[3] and barrel^[4] along with automatic and/or stem winding to store energy, just as in a mechanical watch.^[3] However, the escapement and balance wheel in mechanical watches is replaced by Seiko's Tri-synchro Regulator system, a phase-locked loop wherein a rotor, which Seiko refers to as a "glide wheel", is powered by the mainspring barrel via a stator. The glide wheel in turn powers a reference quartz crystal and accompanying integrated circuit which controls an electromagnetic brake which then regulates the rotational speed of the glide wheel itself.^[3]

The glide wheel is intended to rotate eight times per second; the rotational speed is sampled once every rotation and a variable braking force is continuously applied to maintain that target frequency. As the glide wheel directly powers the seconds hand of the watch, this results in a true continuously sweeping second hand – in contrast to the beats per time motion resulting from the back-and-forth movement of traditional mechanical watches or the tick of typical quartz watches.^[3]

The movement is specified to ±15 seconds per month.^[5]

The design was first conceived by Yoshikazu Akahane at Suwa Seikosha in 1977^[1] and patents were applied for it in 1982;^[5] in total, no fewer than 230 patents have been applied worldwide for this movement.^[6] Initial development was hindered by the high energy consumption of the reference quartz crystal and integrated circuit^[5] making a watch with a then-target 48-hour power reserve impossible;^[7] another attempt in 1993 was also unsuccessful for the same reason.^[5] It was not until a third attempt in 1997, using a quartz crystal and integrated circuit with energy consumption approximately one one-hundredth that used in the initial attempt in 1982,^[7] that a Spring Drive watch with sufficient power reserve was deemed feasible.^[5] Over 600 prototypes were produced during development.^[1][3][8]

The Spring Drive movement was announced publicly in 1997 and presented at the 1998 Basel Watch Fair.^[1][8] In 1999, the first production models were made available in Japan as limited edition, manual-wind watches in both the Credor and Seiko brands.^[2][5][8] The first non-limited model was released in Japan in 2002.^[5]

The 1st spring drive automatic-wind movement of Grand Seiko was released in September 2004, the reference number is SBGA001.^{[citation needed]}

The first automatic-wind Spring Drive model was released in 2005,^[8] and coincided with the introduction of the Spring Drive movement to markets outside of Japan.^[8]

Early models, manual wind and 48h power reserve:

• 7R68 : 30 jewels, date.
• 7R78 : 30 jewels, date.
• 7R88 : 30 jewels, date.
• 7R99 : 32 jewels.

Current calibers with standard features. Time accuracy: monthly rate within ±15 sec (equivalent to a daily rate of ±1 sec) and power reserve (72h) indicator.

• 5R64 : 32 jewels, date, small seconds hand.
• 5R65 : 30 jewels, date.
• 5R66 : 30 jewels, date, GMT.
• 5R67 : 30 jewels, Moon Phase indicator.
• 5R77 : 30 jewels, Moon Phase indicator.
• 5R85 : 49 jewels, date, Chronograph, Izul.
• 5R86 : 50 jewels, date, GMT, Chronograph, Spacewalk.
• 7R06 : 88 jewels, manual winding, Sonnerie.
• 7R08 : 44 jewels, manual winding, Eichi I.
• 7R11 : 112 jewels, manual winding, Minute Repeater.
• 7R14 : 41 jewels, manual winding, Eichi II.
• 9R01 : 56 jewels, manual winding. Power reserve 8 days (192h).
• 9R15 : 30 jewels, date. Monthly rate within ±10 sec (±0.5 sec per day).
• 9R31 : 30 jewels, manual wind
• 9R65 : 30 jewels, date.
• 9R66 : 30 jewels, date, GMT.
• 9R84 : 41 jewels, date, Chronograph.
• 9R86 : 50 jewels, date, GMT, Chronograph.
• 9R96 : 50 jewels, date, GMT, Chronograph.

Current calibers with higher power reserve and higher accuracy. Time accuracy: monthly rate within ±10 sec (equivalent to a daily rate of ±0.5 sec) and power reserve (5 days) indicator.

• 9RA5 : 38 jewels, date.
• 9RA2 : 38 jewels, date. Rear power reserve indicator.