To start, a differentiation should be made between “Heavy Duty” and “High Cycle,” or HICY. Heavy Duty implies a strength or toughness that can be misinterpreted as a design requirement that will handle more cycles. Generally, Heavy Duty terminology when applied to scissors lifts infers a higher load capacity. Some industrial scissors lift manufactures may label their equipment models as “Heavy Duty” starting at 8,000lbs capacity or higher. It does not mean that these lifts will endure any more cycles than the smaller capacity lifts.
High Cycle terminology is described when the application requires continuous cycling of the lift and power unit.
Identify applications that may require a high cycle scissors lift.
Industrial scissors lifts are very robust designs that generally see service lives of 10, 20, or 40 years with minimum regular maintenance. However, some severe applications if operating at a repetitive, continuous cycle can precipitate a premature breakdown of various lift components. This is where HICY design features and options are typically recommended.
Common Failure Modes:
Manufacturer’s typically use intermittent duty motors in the internal power units to minimize the footprint and fit inside the frame of the lift. Due to the inherent nature of a scissors lift cycle this is acceptable. Most lifts take less than a minute to raise (with the motor running) and then the lowering part of the cycle is almost always gravity driven. This gravity down portion of the cycle allows the motor to cool. Additionally, industrial scissors lifts are rarely used continuously; this ensures the motor has more than adequate time to recover from the previous cycle.
When a lift is required to run frequently with short intervals for cooling, this style of motor can over-heat and lose efficiency and fail prematurely. To counteract this problem, the manufacturer will specify a continuous running motor, usually at a higher horsepower.
Continuing with the high frequency example from the last example, it can also lead to oil issues in hydraulic power units. While the oil volume is mathematically sufficient for the application, oil temperatures can easily exceed allowable levels. Hydraulic fluid temperatures above 180°F (82°C) damage most seal compounds and accelerate degradation of the oil. While the operation of any hydraulic system at temperatures above 180°F should be avoided, fluid temperature is too high when viscosity falls below the optimum value for the hydraulic system’s components.
This can occur well below 180°F, depending on the fluid’s viscosity grade. Overheated oil can lead to premature oxidation or thermal breakdown. This in turn may result in sludge or sediment formation which can also cause problems for the gear pump. There have been examples of a plastic tank melting due to the temperature of the oil.
This problem can easily be avoided by sizing the tank appropriately for the application or adding an oil cooler to the system.
Many scissors lifts come standard with DU® style bushings at pivot joints. Technically it is a steel-backed, porous sintered bronze impregnated with PTFE and lead. This style of bushing has high temperature and load carrying characteristics. DU® bushings can be run dry or lubricated to extend their lift. It is an economical solution to the motion and load seen by a scissors lift. Another option for scissors lift bushings is the filament would composite. It has higher load carrying capacity than a DU® bushing at the relatively slow rotational speed of a scissors lift while offering longer life due to the thickness of the liner. These composite bushings do not require lubrication to extend their life. They also tolerate contamination better. So, when an application requires an extended life cycle, or longer periods between maintenance, or may be in a harsh environment, many manufacturers will use the composite bushings.
Most manufacturers use some type of roller at one end of the scissors legs. These are commonly made from medium carbon steel and will use a bushing. A longer lasting, more expensive option is some type of cam follower that is a higher grade (harder) steel and uses tapered, needle, or roller bearings instead of a bushing. Some manufacturers also offer hardened runner strips on which the cam followers run.
Ease of Maintenance:
Another aspect of HICY features is the ability to do maintenance on a lift with little or no down time. These are applications where running every day or 24/7 is not uncommon. Below is a list of common methods manufacturer’s will add to HICY lifts for ease of maintenance.
Besides using composite bushings, some manufacturers offer a removable bushing assembly. This allows maintenance to be done without disassembly of the lift, thus reducing downtime.
Cylinders are retained in the lift by some sort of pinned connection. Most lifts require the pin to be driven out of the clevis or cylinder to remove the cylinder for maintenance. Bolted 2- piece clevises allow the cylinder to be removed more quickly without removing the pin.
NFPA style cylinders
These cylinders are standardized by the National Fluid Power Association (NFPA). This enables cylinders from different manufacturers to interchange within the same mountings. Usually, these cylinders are a national brand which will have local distributors for parts.
External Power Unit
These can feature benefits like an in-line/external oil filter, or quick-access drain plug.
What Constitutes A HICY Lift or “Package”?
Each manufacturer has its own criteria for when a lift application is deemed HICY. Most will look at the number of cycles expected each year. Cycle frequency is also examined.
Generally, modifications a manufacturer makes from a standard lift to a HICY are driven by the elements discussed in COMMON FAILURE MODES and EASE OF MAINTENANCE.
Here is a list of common options that may be seen on HICY models:
• Continuous running power unit
• Higher horsepower power unit
• Larger reservoir (oil)
• Oil cooler
• In-line oil filter
• Composite life-time lubricated bushings
• Removable center axle pin/bushing assembly
• Sealed cam follower rollers
• Hardened runner strips (for the rollers)
• 2-piece cylinder clevis
• NFPA style industrial cylinders
The goal for any scissors lifts manufacturer is to provide years of trouble free operation for the customer. Each manufacturer will have its own definition of what constitutes a high cycle lift and when that option is required. Lift manufacturer’s like Autoquip will consult with you about your application and make recommendations for added more robust features on a high-cycle application.