Innovation aids shaft coupling design

Wednesday 19th July 2017
Innovation aids shaft coupling design

When we look at custom or specialist shaft couplings, it is important to remember that it does not invariably mean a custom or specialist price; it can sometimes actually mean a cost saving.

Shaft couplings by the nature of their placement are ultimately the critical connection between the motor and the driven mechanism; and there is a plethora of shaft coupling designs on the market that match the diversity of that end use.However, true custom solutions rise above just a difference in bore size or how the shaft coupling is attached, for example by set screw, clamp or key way.

Application challenges are numerous and can become apparent at the initial design or post design stage. Other external factors such as cost reduction and supplier optimisation can also warrant a bespoke solution to a challenge. Challenges such as abnormal shaft offset, harmonic vibration, operating environment or just achieving the maximum performance out of the smallest dimensions; all can warrant the design engineer to look at different solutions. However, standard designs, whilst having a legitimate place in the market place can rarely satisfy all demands within the same design envelope.

Clever solutions will potentially encompass, parameters such as a material choice, the ability to change the physical mass within the same envelope size, have up to 90 degrees of flexibility and the possibility to amalgamate parts of the drive system into the one piece part. This is exactly where the Helical beam shaft coupling has the greatest advantage over the competition. It is the ease in which the core product design can be adapted from a standard part and into a cost effective, multi-functional bespoke design. The Helical product pedigree goes back over 50 years and as one of the founders of the technology, the art of altering the characteristics of coiled beam area has been mastered, which ultimately effects torque capacity as well as the angular and parallel misalignment capabilities of the shaft coupling.  The proven design has aided applications around the globe and beyond.

Once the performance dynamics of the shaft coupling are decided, then how the part would best be connected to the drive design is investigated.  In many cases, the final shaft coupling design is normally an amalgamation of separate parts that include such items as external gears, splines, threads and tangs. Most importantly, it enables the design engineer to achieve far greater usage from one item in the total machine, which inevitably leads to greater efficiency. How the standard shaft coupling connectability is enhanced, is really up to the designer. Being smart about the design can save cost.

Unique to the product,  not only can you add more torque transmitting beams to the shaft coupling design, be that double or triple start coil configurations, but also the pitch of the helical beam cut which will add performance, unmatched by competitive designs. The product has a true custom tune performance capability.

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The Heli-Cal product is available Aluminium, high strength alloys, Titanium, MP35N, Delrin to name a few and has been successfully used in medical, energy, military, aerospace and industrial automation.

Application example - Magic carpet fix

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ABSSAC’s customer had an application where a precision encoder was being connected to a carpet manufacturing machine output shaft. The machines are used to manufacture man-made fibre twisted loop pile carpets for domestic and industrial use. If the vibration and movement present at the output shaft was transmitted to the encoder, it would cause signal errors and ultimately prematurely destroy the encoder.  Ultimately this would lead to below standard carpet being produced due to loom operational irregularities. Whilst very little torque was required in the application, torsional stiffness and zero backlash were important design parameters. The customer also wanted a low maintenance solution.

From a coupling design aspect, the application required large axial motion and parallel misalignment because of the assembly tolerances. The customer was originally using a multi-part bellows shaft coupling, but the required angular and axial movements lead to premature failure. Additionally, under certain operating conditions, the bellows coupling allowed too much vibration to reach the encoder. 

The solution was found using an adapted Heli-Cal flexible beam shaft coupling. A low inertia Aluminium part was created using two flexible beam elements in the single piece design. Two integral clamps held the shafts, each flexible element allowed for the maximum angular and axial offsets set by the customer but also maintained a high degree of torsional stiffness.  A "no charge" working sample was then sent for test and evaluation. Testing proved very successful and verified the new Helical coupling design provided better isolation of the encoder from the motor and better performance. Subsequent machines are now being fitted with the low cost, long term solution.

Application example - Amalgamation of parts

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Abssac recently enhanced the operation of a medical orthodontic machine by replacing a universal joint, plastic shaft coupling and a piece of shafting with just a one piece flexured drive shaft unit. Abssac's customer had initially been using a traditional universal joint to connect a hand control, via a piece of shaft and through another plastic flexible shaft coupling, to a small gear box. The universal joint was expensive and was being utilised at the limit of its angular capability, which was causing cyclic torque variants on operation. A separate piece of bar stock was then used to span the drive mechanism to the gear box, which again required 3 degree shaft misalignment, which was accommodated by a small plastic multi-part shaft coupling. The old design was inefficient, time consuming to build and had suffered from reliability issues.

The solution was to take 400mm of Aluminium bar stock, 15mm in diameter, which would provide the length requirement of the drive shaft. Then using the unique ability to adapt the helical beam part of the helical beam shaft coupling, they could provide the two angular misalignments required in the system, but all within the single part. Utilising a double start coil at each end to give the superior torsional stiffness, the drive shaft could accept angular misalignment up to 30 degrees at one end, whilst the other end would accept up to 5 degrees.

The final design incorporated a split clamp design to lock the drive shaft into position onto the driven and drive ends. The new design was then used in 4 other machines using different lengths as the single part provided a zero-blacklash drive solution, which did not require lubrication, maintenance or suffer from operational issues.

Application example - Heat sink or shaft coupling?

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A a classic example of the Helical product’s unique capability. Not strictly a shaft coupling but more a pancake shaped compression spring 50mm in diameter but only 5mm in height. Used to hold an ultra thin reflective mirror on its top surface, in a high wattage industrial laser machining application. High strength aluminium material was used because of its heat transfer characteristics and low cost. By incorporating the unique flexible element into the one piece design allows the easy alteration of the mirrors angle of incidence, whilst maintaining the mirrors position, rigidity and focus.

Application example -Miniature & Medical

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Miniature designs are a growing part of ABSSAC's business as components become smaller and demand more function. A fully functional 3.1mm diameter shaft coupling was recently supplied for a medical application. Using a left and right handed thread shaft for attachment the coupling could accept up to 1mm of angular offset rotating up to 10,000 rpm. The same customer is now looking at a double start flexure with a 1.5mm outside diameter produced in titanium.

Application example - Stress relief elongates life

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With any Helical beam flexible coupling it is imperative that the start and finish of the coil slit is finished smoothly and without scarring or swarf residue, as this will dramatically shorten the working life of the shaft coupling.  The way in which other similar designs are manufactured results in a relatively sharp edge that may act as a stress riser during operation. In certain cases this stress riser is where a crack will form and most likely lead to a complete failure of the part.

Apart from superior manufacturing techniques, all Heli-Cal shaft couplings are shot peaned and hard anodised to further reduce the stress raisers within the coupling to make a the part more reliable. For the more arduous applications, investment in machines that allow the addition of stress relief holes at the start of the coil are employed. The stress relief hole reduces the stress concentration at the coil finish and dissipating it into a smooth transition from the coil end to the solid hub. A stress relief can drastically increase the life of the shaft coupling.

Utilising years of experience and flawless production techniques, the single piece construction totally eliminates any form of friction wear within its design, whilst also ensuring a zero-backlash and no torque loss operation.

Here are some more examples of success.

Mars rover application

Torque choice

Life of shaft coupling extended

Shaft coupling saves fish

 

ABSSAC

Tel : 01386 421005

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