Designers of power transmission drives face the challenge of maintaining market competitive designs while looking at ways to innovate. But they have to balance the age-old question: ?Why change a working drive design??
Replicating a roller chain or gears on a piece of equipment is a safe solution while design changes pose risk and expense.
So where do you find solutions that minimise risk and cost? The answer lies in drive conversions made by customers using equipment or machinery.
DRIVE DESIGN CONSIDERATIONS
Application criteria
Designing a drive system with rotary and/or linear motion begins with some basics, eg:
? Cost (target or market price).
? Performance (speed, torque, power).
? Efficiency (mechanical and electrical).
? Size, weight and space limitations.
? Precision and accuracy.
? Noise and vibration.
? Environment (temperature, contaminants, etc).
? Reliability and service life.
Manufacturing criteria
Consideration is needed for the economics of manufacturing the drive sub-system, eg:
? Cost and accessibility of components.
? Ease of manufacture.
? Assembly time.
? Production line output.
? Warranty.
POPULAR DRIVE DESIGN OPTIONS
The designer of power transmission and motion control drive systems has access to several technologies, eg:
? Roller chain.
? Gears and gearboxes.
? V-belts.
Let?s briefly review each of these systems and then look at examples of conversions which led to improved designs.
Roller chain
One of the oldest, most available forms of power transmission, a roller chain is popular due to its ability to transmit high power/torque at low speed, at relatively low initial cost.
A key disadvantage is that roller chains are noisy and high maintenance. Regular lubrication is essential plus pins and bushings wear cause the chain to stretch or elongate over time. All this causes regular re-tensioning requirements, resulting in downtime and lost productivity.
Gears
Gear drives used in power transmission and positioning applications include such styles as spurs, helical, bevel, planetary, cycloidal, spline or harmonic gears, along with worm gears and rack and pinion gears (converting rotary motion into linear motion).
Gear drives, like roller chains, require lubrication and tend to be noisy. The efficiencies of gears can drop by 60 per cent, meaning loss of torque on large motors.
Belts
V-belts are designed to transmit power via pulleys, sheaves or sprockets. While they are old technology, V-belts are still widely used and have a place in the right application.
Synchronous belts are toothed belts that transmit power by the belt teeth engaging the teeth in a sprocket. They can also provide positioning accuracy for motion control applications. Depending on pitch and tooth type, the backlash ranges from .002? to 0.15?. High modulus tensile cords minimise stretch, allowing for greater precision.
Synchronous belts offer around 98 per cent efficiency over their useful life. They require no lubrication plus minimal elongation eliminates the need for re-tensioning.
The benefits of a synchronous belt include:
? Wide range of power capability.
? High power density.
? Wide speed operating range.
? 98 per cent efficiency.
? No lubrication required.
? Clean running, quiet.
? Maintenance free (no re-tensioning).
? Positioning accuracy.
? Low cost of ownership.
? Widely available.
On a total cost of ownership, synchronous belt drives cost less than roller chains and gear drives.
REDUCED MARKET IMPACT
Poorly performing drives are costly to the user. The equipment may operate inefficiently, raising energy costs, slow the production cycle or cause damage to other components in the system.
Some signs of a poorly performing drive include:
? Frequent component replacement.
? Premature failure.
? High maintenance.
When introduing a new drive design, consider not only the end user?s equipment acquisition costs, but total cost of ownership and customer satisfaction. A drive system with minimal maintenance saves money and increases uptime and productivity.
NEW MARKET SOLUTIONS
Roller chain to synchronous belt
A bottling facility was having glass bottles break at the end of conveyor lines driven by a roller chain. Differing rates of chain wear, stretch and elongation on the multi-stage drives caused speed variations, resulting in one end of the conveying line running faster than the other. As a result, bottles were slammed against each other, breaking and immediately halting production.
When this happened the entire operation had to be shut down to re-tension the chain drives. Additionally, each shift spent an extra two hours of preventative maintenance to keep the roller chain drives running smoothly. Regardless, the roller chain had to be replaced every three months.
This ongoing problem was solved by improving the original design through conversion to synchronous belt drives, using Gates Poly Chain GT Carbon belt systems.
Because the Poly Chain GT Carbon belt doesn?t stretch, the conveying speed remained consistent across all the lines, preventing the bottles from piling up and breaking. Output increased by 20 per cent. Fewer bottles breaking reduced overhead costs plus the two hours per shift of preventative maintenance time was eliminated.
Without metal to metal contact, the synchronous belts are less subject to wear; life expectancy for the synchronous belts is two years, compared with three months for the roller chain.
The annual cost savings from this chain to belt conversion was calculated at $330,000 per year.
Gears to synchronous belts
A gear-driven 30-tonne stamping press operates 24/7 in a manufacturing plant. The continuous hammering action wore down and narrowed the teeth on the 24? bull gears, creating slack in the system, throwing off the stroke timing, and reduced product output. When the motor?s speed variation reached ? 35 rpm, the plant engineer knew that the gears were so worn it was time to replace them, typically every quarter.
The stamping press has a set of gears on either side of the machine, an $18,000 replacement cost. Forty-eight hours are required to replace the gears; that?s two days of lost productivity per machine.
The plant engineer looked for a better solution; the drive had to be compact and strong plus space was limited. He turned to a Poly Chain GT Carbon belt drive system.
After the conversion, the belt drive operated more efficiently than the gear drive and backlash was eliminated. The speed variation in the motor was ? 0.5 rpm, translating into a gain of 15 products per minute. By the end of one week the belt-driven stamping press was producing an extra pallet of products.
As a result of this conversion, the user requested the stamping press manufacturer for additional machines to be redesigned with synchronous drives. The redesign is taking $48,000 out of the cost of each machine.
GAINING A COMPETITIVE EDGE
Field experience is the true test of a drive system. When high maintenance or frequent replacement of components becomes the norm, it?s time to reassess the drive design. As the examples above demonstrate, synchronous belt drives offer one way for your equipment or machinery to gain a competitive edge.
Source: Gates Australia/CBC Bearings and Power Transmission