Miniature DC motors drive tattoo machine design forwards
16 November 2020
Tattooing has grown dramatically in popularity in recent years, driven in part by new generations of tattoo machines that offer increased levels of flexibility, comfort and performance for artists.
Compact, lightweight, portable and increasingly battery operated, these new tattoo machines can be used anywhere, at any location.
All of this places stringent requirements on the motor – the prime mover in the tattoo machine and perhaps the most critical element of the machine design – as Vishal Sapale, Assistant Manager – Design Engineering, R&D at Portescap, explains.
The 21st Century has seen something of a renaissance for the tattoo industry that has swept across the world. Fine artists and increasingly sophisticated technology have redefined what is possible in body art, and what was once regarded as symbolism for countercultures is now very much mainstream for self-expression.
Seeing them on our favourite pro athletes, musicians and celebrities only adds to the popularity of this art form. Tattoos have even crossed over into the fashion and beauty industry. With the millennial generation in particular driving the take-up of tattoos, the industry worldwide is now worth many billions of dollars and is estimated to be growing at over 7% per year, according to research by IBISWorld.
Tattoos are created with machines that use reciprocating needles to repeatedly penetrate a person’s skin at high speed, depositing insoluble ink with each penetration into the dermal layer of skin. Three principle inking strokes – lining, shading and colouring – are used to build up the final image.
For lining, a single needle is used to penetrate the skin. The stroke length, or ‘throw’, need not be great to achieve proper skin depth penetration of the needle. For shading, which generally follows lining, a set of multiple needles is used simultaneously to penetrate the skin. Here, the throw given by the machine needs to be greater than the throw used for one needle lining strokes. The last type of stroke is colouring, wherein multiple needles, in a greater number again, are used. For colouring, the throw needs to be greater than the throw used for shading strokes.
The differences in force speed and depth for lining, shading and colouring means that artists have traditionally used three separate machines; eliminating the need for adjustments of throw, and speeding up the overall process. However, this arrangement is costly in that three machines need to be purchased, maintained and cleaned after each job. New generation machines have a mechanism with added features that can adjust stroke, speed and forces. This enables a single machine to cover all three stroke types, in addition to being effective on all types of skin textures.
Tattoo machine types
The history of tattooing dates back many thousands of years, but the first prototype of an electric tattoo machine as we’d recognise it today was patented by US tattoo artist Samuel O’Reilly in 1891, based on the design by Thomas Edison for an electric pen. There has been a tremendous evolution over the years, and today there are two fundamental operating principles, named for their construction and the way the mechanism operates.
Coil tattoo machines utilise an electromagnetic circuit to provide linear motion to the needle. Generally, a coil tattoo machine has one or more DC coils and spring points coupled with armature bar to move needle groupings. The alternative rotary tattoo machine, in contrast, uses an electric motor with a rotating shaft having an offset cam mechanism at its end to convert the rotary motion of the motor into linear motion for the needle.
Conventional rotary tattoo machines are trigger-activated, in that the cam-slider and motor assembly are perpendicular to the needle-armature assembly. But the latest development in rotary tattooing machines is a pen type design, with the mechanism and needle arrangement in line in a cylindrical casing, offering an ergonomic design for the tattoo artist which is easy to manoeuvre.
The most critical aspect of these new machines is the motor. Not only must it be compact, lightweight and portable, but it must also deliver the required speed and energy density to meet the performance requirements of the three lining, shading and colouring strokes; sustaining all the different reciprocating loads created by the mechanism. Increasingly, the motor choice also must enable the tattoo machine to be battery powered.
Tattoo machines require motors with variable speed and torque, so these design parameters are critical characteristics to consider. Insufficient speed and torque causes vibrations, which does not allow the colouring liquid to spread within the skin uniformly. This is because the pigment does not have the time to be absorbed by the epidermis when the tip of the needle reaches the end position. If the motor has insufficient power, then the artist will have to apply more pressure to the tattoo machine and to incline it. This increases the penetration of the needle and accordingly the pain for the person receiving the tattoo, as well as causing trauma to the skin. It also means increased fatigue for the artist – an important consideration where each session can last several hours.
Motor regulation is the parameter of the motor which defines the speed torque characteristics. The lower the motor regulation, the more powerful the motor. As the torque (load) increases, the speed decreases. If the motor regulation is better, then there is less speed drop with the increase in load. This gives better stability for tattooing, as there is less speed variation with respect to load, resulting in less vibration and noise of the tattoo machine.
For next generation, battery powered machines, efficiency of the motor is also a critical parameter. Higher efficiency will minimise power losses and reduce current consumption, which will increase the battery life.
Further, the motor should be compact in size for proper ergonomics and handling of the tattoo machine. It should also be lightweight to reduce the overall weight of tattoo machine, which will again contribute to reducing artist fatigue during longer tattooing operations.
Because the latest generations of rotary tattoo machines are being developed with additional features, such as the ability to perform lining shading and colouring in a single machine, there are further implications for the motor. Adjusting the stroke length (through a cam change) as well as the speed and the force all impact on the variable load amplitude and variable frequencies on the motor.
In looking at a suitable motor, the designer must also take into consideration the various axial and radial forces generated within the tattoo machine mechanism. The load is typically 3N to 9N, acting either radially or axially on the main bearing depending on the nature of the mechanism. Also, there is constant axial preload of 1N to 3N acting on the bearing.
Given these forces, and the fact that the speed of the motor ranges from 4,000 rpm to 10,000 rpm, the dominant failure mode for motors is the bearing failure due to poor design of the mechanism or the wrong choice of the bearing for the amount of reaction axial or radial loads. The first indicator of a bearing failure in tattoo machines is either an increase in vibration felt by the artist or an increase in the motor noise due to the wear of bearings. In most cases, an increase in vibration is soon followed by an increase in noise until finally, the motor fails.
For a motor to be suitable for use in tattoo machines, then, it must not only meet the stringent performance requirements, but also take into consideration proper selection of bearing to accommodate the reciprocating load on the motor shaft.
Miniature DC motors
Portescap develops miniature DC motors and has become a first port of call for tattoo machine designers looking to ensure the highest levels of performance and reliability in their products. Portescap offers miniature coreless DC motors in 16mm and 22mm frame sizes, well suited for use in tattoo machines.
Along with lower inertia and high acceleration capability, these motors offer improved motor regulation and efficiency, as well as high power density and a good price to performance ratio, all in a compact and lightweight package. Also, given that the joint between the shaft and bearing is critical because of the cyclic axial and radial load of the tattoo machine, Portescap has developed optimised preloading of the bearing and the rigid shaft-bearing joint to withstand these load cycles. Hence, Portescap motors produce lower noise and increased reliability in tattoo machines.
Optimised motor selection can be based on space constraints (available diameter and length) or by the motor performance. Typical Portescap models for tattoo applications requiring variable speed and torque include the 22S78 motor, the 16N78 and the 17N78. The 22S78 motor has the best speed torque characteristics, while the 16N78 motor has moderate performance within the smallest diameter. The 17N78 motor offers moderate performance with moderate length and diameter.
Portescap motors are typically operated at high frequencies of 66-166Hz (4,000 - 10,000 rpm) and can extend to even higher frequencies. Most recently, Portescap has launched its new DCT motors, which can deliver even greater speed and torque performance.
Finally, customisation of different aspects of the motor package is available to meet specific customer requirements, including for the shaft, mountings, connections, aesthetics and more.
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