Vertically elevating loads is a ubiquitous task in engineering, but prevailing over gravity while maintaining safety and accuracy calls for myriad approaches. Leveraging electronics for failsafe backups is one trend; trimming consumption and recapturing energy is another. As in many other motion applications, sensors and microprocessors are also proliferating. However, this technological renaissance is particularly dramatic with regard to elevator, lift, and hoist designs, due to their initial costs.
A perfect example is the adoption of variable frequency drives (VFDs) in cranes and elevators: Hoisting speeds have skyrocketed, and for cranes, capacities exceed 1,000 tons. Explains Jeffrey Iacco, president of overhead-crane manufacturer and distributor IWI Inc., Cleveland, "A lot of older cranes have dated dc controls, but during strategic retrofitting, we fit these with new, more reliable motors and VFDs — the latter allowing for more flexibility in setup and speeds."
Iacco adds that his company has been converting cranes to VFDs for the last decade, because the change allows plants to consolidate operations onto ac power — a far more universal power supply than dc.
Common nonload brake hoists (which leverage motor torque rather than a mechanical lock to lower, hoist, or suspend loads) employ VFDs in either open or closed-loop vector control, depending on the lift's mechanical design and class. Otherwise, VFDs in load-brake hoisting applications are operated by open-loop vector control — which accurately controls torque over a wide speed range, just like dc drive control.
The lack of shaft position feedback necessitates rotor tracking by other means, but eliminating the feedback device, VFD input, and cabling can offset the slight motor-performance loss. This setup does not require dynamic braking, as the load brake controls the rate of decline and absorbs the descending load’s potential energy.
"We notice cyclical swings in the percentage of retrofit versus new crane builds, and lately, 80% of business has been new build — likely due to the improving economy," Iacco mentions. The new units are used in myriad applications. Currently, the company is executing an installation for a packaging company that produces salad bowls and trays; they are also installing a crane for a hydraulic-cylinder manufacturer. "However, all new builds are fit with ac drives at the onset," Iacco concludes.
Elevators are enjoying technological gains as well. "Elevator controls, which for decades were technologically behind the curve, are now pretty close to the curve in turns of their modernity," explains David Fried, vice president of Van Deusen & Associates (VDA), New York and Livingston, N.J.
Originally, elevator drives were either ac or dc rheostatic — or used variable voltage with a motor generator set, what was called the Ward Leonard system. Increasingly, newer elevators and high-speed varieties for high-rise buildings run on ac power instead of dc. "It's true that some large dc machines are driven by semiconductor-controlled rectifiers (SCRs) of various sorts instead of motor generator sets," says Fried. Relay-logic controls are superseded by solid-state components. "Even so, though SCRs are a fairly recent addition to the lineup of elevator componentry — only appearing within the last 20 or so years — we’re already seeing them superseded by Power Factor 1 types of drives that are even more efficient."
Most modern high-speed elevators found in high-rises sport motors powered via variable-voltage/variable-frequency ac (VVVFAC) drives. Says Fried, "Until recently, VVVFAC drives could only drive at about 50 hp at relatively slow speeds of around 350 fpm, but now the newest units can power 200-hp machines moving elevators at up to 2,000 fpm in the highest-rise buildings."
Mechanical considerations for hoists
On cranes, the mechanical gears, wheels, and bearings can last decades — even if the runways or trolleys require replacement. According to Iacco, where cranes are repurposed, their relocation from one bay to another typically requires a resizing of spans — which can require on-site reconfiguration, welding, and so forth. Even so, older trolleys with cast-iron frames, drums, and gear cases can be replaced with modern, compact units, and in many cases, boost crane capacity with trolley-weight reduction alone. Modern precision gearing can also reduce vibration, wear, and noise common in older cranes, while flux vector control boosts functionality. However, Iacco advises, "Plant engineers leading a repurposing or new crane purchase must have a reasonably accurate estimation of what the crane's duty cycle will be. Will the crane be used infrequently, or will it function as part of a production line operating 24 hours a day?"
Swing during lifts
Where motor-driven systems abound in permanent installations, free-range forklifts often make use of hydraulic drives for hoisting, and rack and pinions for concurrent guiding. How is accuracy maintained? In one case, Novotechnik U.S. Inc., Southborough, Mass., supplies RSM2800-Series multiturn angle sensors to The Raymond Corp., Greene, N.Y., for its 9000 Series Swing-Reach rotating forklifts. Unlike normal varieties, for which forks swing around when an operator turns the lift, the 9000’s forks swivel to point forward or sideways. This feature lets the forklift squeeze into narrower storage aisles (even down to 66-in.) and lift 3,000-lb loads by 45 ft.
For controls, the 9000 uses Raymond’s Intellispeed control and ACR system, which coordinates axes’ speeds based on load, and smoothes lift-and-lower speeds. The lifting forks vertically travel on a mast with hydraulic cylinders and steel rails. However, Raymond engineer Dan Driscall explains, “This system swivels and travels back and forth, so for stability we must track the center crossing and approaches to end points. The mast and forks move with rack-and-pinion steering, requiring multiturn measurement.”
To this end, the noncontact RSM2800 sensor measures angles to 5,760° for position and dynamic tracking, even down to 0.078 in. The sensor can also track position and turn counts without power using giant magnetoresistance. Here, when magnetic layers inside the unit are parallel, resistance drops; as they turn out of this alignment, resistance increases — as does its value used to measure position. One detector measures rotation angle (converted to voltage output by controls), while another counts turns.
Lifting dangerous loads — a tad
In another heavy-load application — nuclear-fuel transport — lifting capabilities for finessing objects into place is more important than compactness. Consider this: Transnuclear Inc., Columbia, Md., has developed what one might call prefabricated long-term housing for the storage of spent nuclear fuel. Called NUHOMS, these concrete abodes incorporate layers of lead, steel, and helium in cylinder-shaped docks that accommodate spent-fuel casks. How are the casks inserted into their final resting places? Wheelift Systems of guided-vehicle manufacturer Doerfer TDS Automation group, Waverly, Iowa, makes cars for these casks, and the company's Self-Propelled Modular Transporter (SPMT) can even augment or eliminate heavy lifting equipment such as cranes. Much like the Raymond 9000 forklift, this vehicle is a multitasker: Omnidirectional steering from the Wheelift coupled with a 10-in. lifting capability allows one remotely located operator to align casks being deposited in NUHOMS docks to within 1/32 in., and eliminates independent leveling and alignment.
Constellation Energy's Ginna Nuclear Power Plant in New York is the first to use Wheelift transporters in their NUHOMS spent-fuel storage operations.
Granted, escalators aren't as hazardous as spent nuclear fuel — unless you believe the warnings put on pairs of a certain popular rubber clog. Even so, the reliability of people lifts is paramount. For old units being retrofitted, most developers, property managers, and engineers upgrade when an elevator's downtime, repair costs, and complaints increase. As John Powers of Century Elevator Inc., Quincy, Mass., explains, frequent repairs can lead to a feeling — real or imagined — that safety is being compromised. "We also get upgrade calls if there’s electrical noise — mainly a nuisance to those working nearby — or vibration, which can damage the structure itself," adds Powers.
Sometimes, historical elevator upgrades require creativity. Last year, when a Boston nightclub owner wanted to refurbish a glitzy elevator, the Century Elevator team overhauled the circa-1900 unit with a custom redesign of mechanical and electrical inner workings. "However, the older a unit, the more chance we may need to jury-rig parts from other sources," Powers explains. More typical projects involve standard repairs and regular installations in offices, residential complexes, retail sites, and warehouses.
ASME dictates the national safety code for the United States — A17.1 — for new elevators and escalators. "Local city and state jurisdictions adopt that code with modifications — such as New York City's Appendix K, for example — which ensures that elevators adhere to both ASME and the city’s building-code requirements," adds Fried of Van Deusen & Associates.
In addition, ASME 17.3 for existing elevators and escalators establishes retroactive safety requirements and inspection schedules. According to a representative of Schindler Elevator Corp., Morristown, N.J., common traction or "roped" elevators require one low-speed, no-load test each year, and a full-load, full-speed test every five.
Standards do not change with technology. To the contrary, codes and safety requirements are performance based. ASME guidelines are phrased, "A means will be provided to control the speed of the car" or "A means will be provided to remove power from the car in the event of overspeed," but these standards do not dictate minute details of an elevator’s inner workings — or the means and methods to execute these functions.
Now, the sophistication of newer equipment allows for diagnostics, remote monitoring, and overall troubleshooting capabilities, which can contribute to higher safety for elevators; reliability has also improved significantly.
"By the same token, for safety and to satisfy ASME requirements, all elevators must have a friction brake able to hold 125% of the load on a passenger unit, and the car must level and stop in each direction, even when carrying 125% of rated load," explains Fried.
On modern equipment, these brakes are normally just holding brakes, and not a means for slowing — so they're applied only after the elevator has stopped. The only elevators that use brakes for slowing are old single-speed ac or dc rheostatic cars found in legacy apartment houses or loft buildings, for example — and their leveling capability is marginal.
While regional directives apply to regular passenger elevators, application-specific requirements apply to specialty lifts and hoists. Consider DIN 56950, a European entertainment-technology safety directive. This standard spells out requirements when stage lifts are constructed for international sale.
As Ann Marie Fortunate of Serapid Inc. explains, "On full-stage lift systems with platforms, there are additional safety precautions taken — including installation of an astragal safety edge, which has sensors for obstructions in the lift's path. Controls are also programmed with deadman switches on remote pendants. With these, the pendant button must be held by the operator, and if released, the system stops." Often, Serapid also designs interlocks into the controls, preventing stage lifters from moving if access doors are open, and locking these same doors if the platform isn’t in a safe position.
For industrial hoists and cranes, two groups establish safety standards for lifts and hoists. Iacco of IWI explains, “It is the manufacturer's responsibility to ensure that machinery meets applicable codes — namely, those of the Crane Manufacturers Association of America (CMAA) and Hoist Manufacturers Institute (HMI). Components must be sized to safely raise, lower, and hold loads. A design’s drive and motor must output adequate shaft torque for the mechanical design and CMAA/HMI service class.” Typical guidelines require at least two means of braking for most setups.
Electromechanical hoists, especially ubiquitous nonload brake hoists, must prevent overspeed and travel. Sometimes open-loop drives are paired with mechanical overspeed protection, or encoders output shaft speed feedback to warn the VFD of overspeed. Dynamic braking, which dissipates energy via the VFD and out to resistors, is often counted as a brake.
Give your crane a (modern) brake
While we’re discussing brakes for safety, consider this: Eliminating inefficient holding and friction brakes can actually improve performance while reducing wear and allow gearing and other components to operate more coolly. Aaron Kureck at Magnetek Inc. underscores that modernizing, retrofitting, and repairing cranes requires the replacement of outdated or unserviceable crane brakes. Here’s why:
• New brake performance is improved, and brake linings last longer, particularly when used in conjunction with ac adjustable frequency controls or digital dc controls. In many cases, their pivot-arm journal bearings are replaced with self-lubricating composite bearings, which resist shock, contaminants, and corrosion.
Some newer brakes even adjust automatically to compensate for lining wear and balanced brake pad wear — for extended brake life.
• Old hydraulic brake systems, which tend to leak, can be replaced with modern brake-by-wire packages incorporating foot-pedal-operated ac thruster brakes that still “feel” like hydraulic brakes to operators.
• Kureck also points out that complete "drop-in" brake packages meet performance and dimension characteristics of original-equipment brakes — often at a lower cost than replacement components — with significantly shorter lead times. They can operate with existing brake wheels and avoid costly brake-support modifications.
• Heavy-duty caliper disc brakes with ratings from 50 to 30,000 ft-lb can replace existing brakes in high-duty-cycle, high-speed, or high-torque stopping ac or dc applications.
• Finally, low-cost ac thruster brakes often replace aging dc drum brakes. These eliminate the need for ac-dc rectifier panels, and some support stepless, externally adjustable time delays for both brake setting and release; external torque springs permit maintenance personnel to dial-in just the right amount of stopping torque for traverse motions; and automatic adjustment and automatic equalization reduce maintenance costs.
Boosting escalator efficiency
Power Efficiency Corp., Las Vegas, takes another tack to improving lifting applications: You guessed it, efficiency. The company designs and sells motor controllers that reduce energy consumption in ac-induction motors.
When induction motors are lightly or variably loaded, they tend to be inefficient, because they’re designed to run most efficiently at high loads — and motors in escalators are no exception. These big units are designed for heaviest-case scenarios, in which passengers populate every single step, but this rarely happens (except, for example, at closing time during a McCormick Place, Chicago, tradeshow) so the motor is usually lightly loaded and wasting electricity. In fact, most normally run at under 40% of full load — squandering millions of dollars annually to run lifts and elevators at hotels, airports, transit stations, and even industrial mining and plastics facilities.
Power Efficiency's answer to the problem — E-Save Technology — is a three-phase motor controller that is more like a soft-start (that continually monitors load, not just at startup) than a VFD. How? VFDs control motor speed by changing voltage and frequency. In contrast, Power Efficiency's controller maintains line-voltage frequency and speed while shaping input power.
More specifically, its algorithms monitor motors and provide just enough power to perform a given mechanical task. In short, voltage to the motor is reduced under light loading with a dip in magnetizing current (and iron losses) during each ac half cycle. This also cuts the power’s inductive component to increase the power factor. Meanwhile, a fast-response algorithm stands ready to apply full power to the motor in case of sudden load increases. The units reduce energy use by up to 35% in constant-speed applications, and large escalator and elevator motors benefit. On 40-hp escalators at Caesars Palace Hotel and Casino in Las Vegas, for example, Power Efficiency’s motor controller lowers average kW use from 6 to about 4.
In fact, Powers of Century Elevator concurs on the value of efficiency. After safety, facility engineers’ second most common reason for initiating elevator upgrades with his company is to reduce energy costs. Here, he argues that a new unit is usually more efficient. However, there’s a caveat: On newer buildings, elevators become obsolete more quickly, because according to Powers, new solid-state gear and microprocessors have shorter life spans than older relay-logic equipment.
Potential energy: Use it if you got it
It's not just finessed input that can reduce energy expenditures for lifting and hoisting; regenerative braking in these vertical applications makes use of gravity to wring some of the unused output from a system.
As in horizontally oriented motion designs, regeneration occurs in vertical lifts and hoists when the load overhauls its driving motor and causes it to function as a generator — typically, when the load descends. Then, the motor generates electricity that flows back to the inverter dc bus, causing its voltage to rise.
Reconsider the Raymond Corp. forklift discussed earlier — a sophisticated version of an otherwise workaday vehicle. The design iteration recovers energy as it lowers loads, just as most nonload brake hoists can regenerate.
Here's the catch: If the energy from a motor being overhauled by the load is not removed, the inverter overvoltages. Dynamic braking is one option that removes regenerated energy by turning it into removable heat with resistors. Another more efficient option is the integration of a regenerative drive that recaptures this energy by turning it into three-phase 60-Hz power that’s re-fed to the power supply to reduce total electricity use.
One new example of regenerative drives is the fully automatic logistics center of B&R Automation Corp., Roswell, Ga., at its global headquarters in Eggelsberg, Austria. Ultra-modern storage and retrieval machines (implemented by TGW Systems Integration GmbH) leverage control, visualization, and drive technology from B&R to cover total warehouse space of 6,600 m2, a shelf height of more than 19 m, a shelf length of 72 m, and a capacity of 18,000 palettes.
Five aisle-guided storage/retrieval machines run down two-sided shelf aisles for 150 double-actions/hour: 300 euro palettes can be precisely stored or retrieved per hour. Euro palettes weighing up to 1,000 kg are precisely positioned over 72 m at 2.5 m/sec2. The energy efficiency is an important advantage. Power regeneration on the B&R servo drives reduces energy costs during daily operation by about a third.
Wedges in smaller lifts
The smallest, most delicate tasks — manufacturing pharmaceuticals and semiconductors — require lifting of a completely different flavor. Typically, assembling or testing in these applications requires strokes not much longer than an inch, though adequate resolution is needed. Interestingly, transverse motion is used to accomplish quality vertical positioning.
In one lift from IntelLiDrives, Philadelphia, a wedge-shaped base is paired with a ballscrew to elevate payloads (to 10 kg) by up to 5 mm. The wedge rides on precision bearings to convert horizontal motion (from the ballscrew) into true vertical elevation. Stepper or brushless servomotors can drive the design. What’s a typical application? This MLVS lift might bring a chip under testing into alignment with a vision system for inspection, for example. Repeatability is to 0.5 µm and resolution is to 20 nm.
However, ballscrews in such designs can take up significant vertical space — and while similar leadscrew-driven wedge lifts are smaller, backlash and screw windup limits resolution to about 50 nm. Therefore, IntelLiDrives also offers a linear-motor air-bearing lift stage for more demanding applications — as does Aerotech Inc., Pittsburgh.
In fact, Aerotech’s ANT130-5-V linear-motor-driven wedge-style vertical lift stage is yet another option — this one delivering astonishing 2-nm resolution (the minimum incremental step size) for nanopositioning. Here, the wedge incorporates two sets of anti-creep crossed-roller bearing linear guides — one in the horizontal plane, and the other inclined at a 1:10 slope — to convert horizontal wedge motion into vertical tabletop motion. The incline also provides mechanical advantage to increase motor payload capacity. Travel reaches 5 mm at speeds to 75 mm/sec, compared to less than 1/100 of that from ballscrew/stepper-driven units designed for other applications. The lack of hysteresis or backlash also enables accurate and repeatable nanometer motion, “useful in fiber alignment, optical delay element actuation, sensor testing, and smooth scanning processes,” explains Stephen McLane. Multiple axes combine for flexible designs and multi-axis configurations.
Wedge-based lifts can elevate heavier loads: Aerotech’s AVS1000 stages incorporate a machined wedge that converts horizontal movement into small incremental vertical movements. This positions loads to 135 kg with resolution to 5 nm, and by centering the payload over the bearings, there’s no cantilevering.