First of the Mohicans

Written by: Tim Fryer | Published:

A novel design could revolutionise the torque coupling world dominated by universal joints and gear couplings. Tim Fryer found out more.

Sometimes a good idea spawns another, then another and soon that idea turns into a whole range of products or technologies. This is the case at Punk Couplings, where the quest for lightweighting a downhill trike resulted in the development of technology that may have applications wherever a system has parts that move relative to each other. This means anything from the mechanism in a photocopier to the drivetrain on a cargo ship.

Bored skiers, thought Simon Parker, needed some high-speed downhill thrills when the snow was scarce, so he started working on a high performance downhill human powered Tadpole Trike. Prototypes showed the need for lightweighting. Not to increase speed, because it already reached 75mph, but to provide control at such high speeds. Parker explained: “They had a very unresponsive suspension. I found if I was going pretty fast down a mountain and one of the front wheels hit a rock you would take off and rotate axially thus landing on your face.So I'd almost parked the design then because it wasn't going to be a safe vehicle, it was going to be a widow-maker.So I had a think about that, and this is where the Punk story starts.”

Much of the weight was in the steering hub so Parker, initially just for the purposes of making a better trike, developed Punk Hub centred steering. “What we've got here is male and female nesting ring with spherical surfaces and loading slots,” described Parker. “Then the cylindrical key ways which we call mohicans because they look like the punk haircut. They control the plane of movement.”

It allowed +/- 15o movement of the front wheels, more than enough for a high speed vehicle, and an assembly that weighed a quarter of the one that it replaced. Parker said: “It gave us a much more responsive suspension due to the reduced unsprung weight and so the wheel can change direction very quickly.”

It then occurred to Parker that adding a third ring with a plane of movement at 90o to the other effectively created a universal joint that could be used for torque coupling. Given the vast number of applications for torque couplings, suddenly trikes were not worth trifling with and the focus over the past couple of years has been to refine the design to build in sealing and lubrication.

The technology has evolved into two core streams. One is the Punk Prime, which is the original concept of the three nested rings guided by the mohicans. The second stream is the Punk Gim-Ball. Again its core components are three nested spherical rings but the mohicans are replaced by axles to produce an equivalent pair of rotating axes, again at 90o to each other in the same way as the mohicans are in the Punk Prime.Patents for both the Punk Prime and the Punk Gim-Ball have now been granted and Punk are seeking to licence its technology to manufacturers.

In the Punk Prime the pressure is distributed across a relatively large surface area and so it can be used in high-torque but lower speed applications, but the consequence of this large surface area is that bearing pressure is low but the surface velocities are high due to the distance from the centre of rotation. Conversely the contact point at the axle of the Punk Gim-Ball is much smaller, so it is not capable of as much loading but can move faster than the Punk Prime.

Parker claimed: “We have a combined axle and mohican solution, which we believe is the world's first waiting fail safe geometric coupling.” Non-geometric options are available that rely on the deformation of materials, like bending springs or elastomerics, but this is the only one with perfectly fitting geometry.

Failsafe is important in such sectors as aviation, where a monitored gradual part failure is appreciated more than a single, unexpected catastrophic failure. Parker explains: “This particular failsafe has been weighted in the direction of high-torque low speed, so we've made the mohicans the primary torque. In aerospace we have a term called damage tolerance, which means can it break and still carry on operating?We've got an interesting case here, which is unique we believe, in that we could lose one mohican and it would still carry on operating with just the other one because the spherical surfaces then start taking up that kick load. If we then lost the second mohican, we can tolerate that damage because the axles come into play.We could lose one more axle and it would still carry on operating with just one axle.”

A PunkNut leadscrew assembly

The largest application areas basically fall into either an alternative for a universal joint, or as a new form of gear coupling, but both are essentially transmitting torque. Early target markets are industrial, aerospace, automotive and marine but it extends into any application where there is a mechanical (internal combustion engine, electric motor or rotary actuator) motor driving something with rotational movement.

In static rotating equipment such as industrial machinery, factors such as manufacturing tolerances, vibration, thermal expansion, structural flexing and wear require the misalignment to be compensated for by a flexible torque coupling. In dynamic systems such as a vehicle these factors still need to be considered, as well as the range of movement/travel required by the driven elements i.e. the wheels or axles.

Parker worked in large passenger aircraft design in the past and during his involvement published 19 patents, two of which have found applications in the newest models. It is no surprise that one of his first target markets is aerospace, even though he concedes that, due to the type of product and certification procedures, ‘talking to aerospace can be very frustrating.’

This early stage study is involved in the deployment of flaps on the trailing edge of aeroplane wings. Wings bend up and down in flight depending on how much fuel they are carrying, how much lift, where they are in the flight cycle and so on, and the resultant angle of misalignment needs to be catered for in the flap control. Parker looked at solving this problem with Punk’s couplings. He said: “Our analysis showed we could potentially save several kg’s per aircraft on the first pass.We were encouraged by this large passenger aircraft manufacturer to go and talk to their Tier 1 suppliers, which we are currently doing.”

The analysis mentioned would have been the output from the company’s ‘Punk Coupling Sizer and Analytics Software’, which is capable of automatically sizing and performance analysing a Punk coupling for any given application. The software outputs component dimensions for specified variables such as shaft size and maximum misalignment angle. User defined options on interface type (key or spline), materials and sealing methods are automatically incorporated into the design.

Having created what it calls the Punk geometric philosophy, the problem becomes focussing on a manageable product range, as applications and variations are abundant. For example, the ‘double Cardan’ coupling to deliver constant velocity through a miss-aligned drivetrain can be resolved with a ‘double Punk’ – two Punk Primes connected with a common sleeve.

Other variations include the PunkNut, which applies the philosophy to leadscrew and ball screw nuts in linear actuators as a means of accommodating angular misalignment between the leadscrew or ball screw and carriage rails. In this application the inner surface of the inner ring has the lead nut thread or contains a ball screw nut. The axial load is carried by the spherical surfaces and the axles purely react the torque of the leadscrew and nut or ball screw and nut. Simple linear slides allow movement, which is particularly useful when accommodating variations in a system.

For example, one large boat manufacturer was looking for a better way of controlling a moveable sundeck. The parallel guides were on either side of the boat at some distance apart, and set up was difficult because tolerances were miniscule. What’s more the material of choice in such environments is GRP, which will flex. PunkNut in this application requires less accurate installation thereby reducing assembly time and is more forgiving. The resulting mechanism will deal with the structural flexing of the system it's bolted to.

“This is a completely unique, a new product which seems to have set the industry alight,” claimed Parker. “We showed a demonstrator to a group of manufacturers who’ve been doing this for 30 years and they couldn't believe what they were seeing.This is just an example of one of the spin-off products from this geometry. It's not just torque couplings which is the message I'm trying to get across.”

Compared to Universal Joint

Increased torque transmission capacity

Reduced moving parts and overall parts count

Reduced space envelope

Increased torque density (i.e. reduced weight)

Compared to Gear Couplings

Increased misalignment capability – up to 15o

Zero/low backlash

Axial load capacity

First test

Parker tested several early Punk prototypes in his garage using a lathe to power the rotation a pair of Punks connected via a misaligned driveshaft. A motorcycle hydraulic disk brake with a weight attached to the master cylinder provided the load and this had to be cooled with water to stop it glowing red hot. It successfully ran for about 10 days to reach a million cycles. This test was critical in designing the self-lubricating function of the Punk.

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