Hoists and Load Lifting Accessories
Hoists and Load Lifting Accessories
Lifting and moving of heavy materials from one point to another point in a manufacturing plant requires the use of specialized machinery. Load lifting equipments make material handling easier for the working people. Crane is equipment for the lifting or the lowering a load, and moving it horizontally, in which the hoisting mechanism (hoist) is an integral part of the equipment. It can be driven manually or by power and can be fixed or mobile equipment. It is the prime load lifting equipment and is essential equipment in any production shop. Cranes are used for the task of lifting materials both for the production as well as for the maintenance in the production shop.
There are also other hoisting equipments besides cranes which are normally needed. A hoist is used when elbow grease and muscle is not enough because of the weight of the object to be hoisted. Hoists assist workmen in the shop floor for loading, hauling, and unloading heavy materials in those places which are not in the reach of the crane.
A hoist is simply equipment which is used for the lifting and lowering of heavy loads. It is equipment for raising or lowering a load, but does not include a car or platform riding in guides or horizontal motion. A hoist can be a fixed hoist or can be an integral part of a crane mechanism. In most applications, the hoist is normally suspended overhead by means of a hook, lug, or trolley. A hook or lug can be used to permanently mount a hoist in one location, or affix it to a trolley. A trolley is a vehicle which travels on an overhead rail or beam, normally used to move the hoist and its load from place to place.
Hoists lift and lower by use of a chain, cable, or wire rope. The lifting means for suspending and moving the load up or down in a hoist are normally link chain (coil chain), roller chain (similar to bicycle chain), or wire rope, but other materials such as high strength fabric, can be used in specialized applications.
Hoists and trolleys can be powered by hand, electricity, air, or hydraulic power and are available in various shapes and configurations. Most of the commonly used hoists are either manually operated or utilize electrical power. The lifting capacities depend on the construction of the hoists. The travel of the equipment is generally directed by an operator, either manually or with a wired pendant station or wireless controls.
A hand chain hoist is a type of manual hoist powered by a hand chain to lift or lower the load. It is a manual hoist which is a force multiplier. It gives a workman the ability to lift very large loads (upto 50 ton) with ease by using mechanical advantage. Most hand hoists are used for infrequent maintenance applications where speed is not a requirement. They are considerably less expensive than powered hoists, but they require physical effort (pulling of the hand chain) to lift the load. They are not fast and are not generally specified for continuous lifting applications, especially when long lifts are required.
Manual hoists lift a load under the direction of an operator. The operator typically raises and lowers a lever to activate a ratchet and pawl configuration which incrementally lifts or lowers the load, or to apply or release tension. Examples of manual hoists are a lever hoist, ratchet lever hoist, ratchet hoist or wire rope puller. In a production shop, hand chain hoists are most often used as an in-plant tool for periodic maintenance applications. Examples of manually operated chain hoists are given in Fig 1.
Fig 1 Manual chain hoists
Parameters to consider when specifying hand chain operated hoists include (i) speed of the lift, (ii) height of the lift, (iii) frequency of the lift, and (iv) weight of the load. Several hoist manufacturers also provide overload protection devices for hand powered hoists, either as standard equipment, or as an added cost option. This device protects the user, the overhead structure and the hoist from an ‘excessive overload condition’. Many of the hoist manufacturers use a friction type, clutched hub, as a part of the hoist’s hand chain wheel. When the pull on the hand chain is large enough to slip the clutch and prevent the load from being lifted, the operator becomes aware that the hoist is overloaded.
The capacity of the hand chain hoist is the maximum weight of the load which can be lifted and is usually given in tons. The rated capacity of the hoist is the maximum load for which the hoist is designed by the manufacturer to lift. The lift is the maximum length of travel required for raising and lowering the load and is given in metres. The hand chain drop is the length of chain required for the hand chain of the hoist. The drop is normally around 600 mm less than the length of the lift, which prevents the chain from contacting the floor. The actual length of hand chain needed is around 2 times the lift minus 600 mm, as the chain is in a continuous loop when it travels around the hand chain wheel of the hoist.
The hoist suspension means how or where the hoist is attached. Most of the hand chain hoists are with a top hook used for single point suspension. They can also be hooked on to a trolley, or built special with a top lug or eyebolt. This allows attachment to a low headroom style trolley for more permanent installations.
Headroom dimensions are important for those applications which require operation in close quarters (low ceilings, obstructions, and short lifts etc.). Headroom dimension is the distance between the bearing surfaces of the upper and lower hook at the high hook position (or the minimum distance between the hooks).
Manual hoists can be trolley mounted. Trolleys can be of several different configurations. The most common are the hand powered (plain or push type). Also trolleys can be hand geared (hand chain driven) and motorized (electric or air powered). A push type trolley is normally used for moving light loads and if load spotting accuracy is not critical. The trolley is moved along the beam by pulling the hoist’s load chain, usually in the lower hook block area. Hand geared trolleys are used for heavier loads, load spotting accuracy, or if the trolley is mounted very high. Powered trolleys are often used for heavy loads, longer distances, repetitive applications, or where faster speeds are required.
Lever operated hoists are sometimes referred to as ‘come-a-longs’ or ‘pullers’. They are used for stretching, pulling, tightening or positioning loads, but also have the ability to be used as a hoist in some applications. The lever hoist design is similar to that of a hand chain operated hoist. Instead of using hand chain to move the load, the hand chain wheel is replaced with a lever or handle. Reciprocating operator input through the lever moves the load in small increments. Speed is not a characteristic of a lever operated hoist.
Electric hoists are normally used in the production shops for the raising, lowering and transporting material throughout the shop and positioning components in process or assembly operations. Electric hoist is powered by electrically driven motor. It is easy to operate and offer more flexibility. It is often specified when the application calls for more frequent and faster lifting, such as on a production line. An electric hoist with a motorized trolley is ideal for repetitive lifts which are required to travel long distances. A wide range of capacities and speeds are available for both hoists and motorized trolleys.
Mechanically, an electric hoist is very similar in design to a hand hoist. To put it simply, physical operator input (force applied to hand chain or lever) is replaced by an electric motor and controls.
In electric hoists, controls are needed to start and stop the hoist motor and reverse its direction. A push button pendant station is normally used to control the hoist and/or trolley motions. Electric powered hoists are available in hook, lug or trolley (push type, hand geared or motorized) suspensions. The electric hoists lifting medium can be either chain (coil or roller type) or wire rope. Wire rope hoists are generally used for higher capacities, faster speeds, smoother and higher lifts. However, chain hoists with the same capabilities as wire rope hoists have become increasingly popular in recent years. Coil and roller chain hoists are the most popular in the 3 ton and smaller sizes because these hoists are generally smaller in size, lighter and less expensive to manufacture.
All chain electric hoists provide true vertical lift (load does not vary from the hoist centerline during hoisting or lowering). Specially constructed wire rope units can also be furnished to provide true vertical lift when required. Chain and wire rope electric hoists are shown in Fig 2.
Fig 2 Chain and wire rope electric hoists
Important parameters required to be considered while specifying overhead electric hoists include (i) speed of the lift, (ii) frequency of the lift (iii) lifting height, (iv) weight of load, (v) duty cycle, and (vi) power supply available. Duty cycle is one of the most important considerations when choosing an electric hoist for a particular application. Duty cycle is generally defined as ‘a specified number of operations at stated intervals’. For an electric hoist, duty cycle is how much work the hoist must perform in a given period. The weight of the load, lifting distance and frequency of operation, all play an important role in determining the duty cycle capability of an electric hoist. The Hoist Manufacturer’s Institute (HMI) has classified duty cycle requirements by hoist class. The hoist classes are H1 (infrequent or standby), H2 (light duty), H3 (standard duty), H4 (heavy duty) or H5 (severe duty).
Electric hoist is normally equipped with overload protection devices as standard equipment. The overload protection devices are based on the use of (i) internal friction type clutches, (ii) electrical shut-off devices, or (iii) low torque hoist motors.
Parameters required for specifying electric hoists are (i) capacity in tons, (ii) lift in metres, (iii) lifting speed in metres per minute, (iv) lifting medium i.e. chain or wire rope, (v) power supply voltage, (vi) control voltage, (vi) push button cord length, (vii) power cord length, (vii) abnormal operating conditions such as corrosive, outdoor, dusty/dirty, extreme temperature ranges, hazardous locations, etc., (viii) duty cycle requirements, (ix) type of suspension such as hook (swivel or rigid), lug, trolley (push, hand geared or motorized), (x) headroom requirements, (xi) trolley hand chain drop for hand geared trolleys and trolley speed for motorized trolley, and (xii) beam size and type on which trolley is operate on.
Pneumatic and hydraulic hoists
Pneumatic hoists or air hoists are powered by pneumatically driven motors. When equipped with a chain as the lifting mechanism, they are called pneumatic chain hoists. Likewise, when equipped with a wire rope as the lifting medium, they are called pneumatic wire rope hoists. These hoists are often used in environments which need electric spark avoidance due to a potentially explosive atmosphere. Hydraulic hoists are powered by hydraulic motors.
Load lifting accessories
Load lifting equipments are used with a number of attachments (accessories) to facilitate lifting of the load. These are end effectors or below the hook equipments. These consist of a variety of different, application-specific, attachments which can be added to the hoist to handle the lifting or positioning of different loads.
Lifting accessories are the components or equipment which are not part of the load lifting equipments but are either attached to the lifting equipment or load to be lifted and are placed either between the lifting device and the load or on the load in order to attach it. Examples are slings, spreader beams, lifting beam, magnet, lifting eyelet, clamps, shackle, C hook, gripping lifters, lifting forks, and crane scales etc. Some of the load lifting accessories are described below.
Slings – These are accessories used for lifting and transferring of loads with the help of cranes, telphers, or hoists. Slings enable in the activities related to create anchors, attach to loads, lift loads, pull loads, and lower loads. The dominant characteristics of a sling are determined by the components from which it is made. Slings are generally one of six types namely (i) chain, (ii) wire rope, (iii) metal mesh, (iv) natural fiber rope, (v) synthetic fiber rope, or (vi) synthetic web.
C hook – It is a device which enables the lifting of a coil through the insertion of the hook into the coil’s inner diameter. A motorized hook rotator powers the rotation of the hook attached to the bottom block of a hoist for additional load control.
Eyelets – They are intended to be placed on the load either by threading or by welding for lifting it
Gripping lifters – They use either friction or indentation-causing pressure to hold a load. Tong grabs or clamps utilize a scissor-type action to grip a load. Coil grabs grasp the outer diameter of a coil via tongs or gripping mechanisms to lift or turn it.
Mechanical lifters – They are composed of two or more rigid parts which move in tandem when manually actuated to secure the load.
Vacuum lifters – These are equipment which includes one or several suction pads operating by vacuum. They utilize an electric-powered extraction pump and sealed pads to create a vacuum to attach the lifter to an object.
Sheet lifters – They use two claws to grab a load of sheet metal or wood by wrapping around the edges. A lip on the lower portion of the claws prevents the sheet from falling out of the lifter.
Pallet lifters – They use forks to lift pallets from underneath.
Lifting beams – They are made from solid or fabricated metal, or from wood and are suspended from a hoist/crane to provide multiple load lifting points for better security and control of the load’s movement. A spreader beam uses two or more hooks to spread the load over more than one lifting point.
Magnets – They are equipment with a magnetic field creating sufficient force for gripping, holding and handling loads with ferromagnetic properties. They lift, carry or release flat or round ferrous objects with or without an electrical power supply.
Clamps – They are non-powered equipment used to handle steel plates by clamping them between the jaws.
Lifting forks – These consist of two or more arms fixed to an upright with an upper arm, essentially to lift palletized or similar loads.
Crane scales – They consist of dynamometer placed between lifting hoist and the load to indicate the weight of the load.
Hoisting equipment is used in a variety of areas to support processing and handling throughout a facility. Typical applications include in manufacturing, warehousing and construction to lift loads in support of production or storage activities, loading and unloading, or transferring of material from one process to the next process. Typical applications include (i) during assembly for moving of products through production processes, (ii) during positioning for securing a component for additional work, (iii) during transportation for loading finished products onto open trailers or rail wagons, (iv) staging for holding work-in-process for additional production processes, (v) storage for transporting heavy items to and from storage areas, and (vi) warehousing for moving large, heavy products to and from one place to other place.
Hoisting equipment provides a variety of benefits as given below.
Customizable – Hoisting equipment can be customized with below the hook attachments, end effectors or specialized tooling to handle a diverse variety of products and loads.
Ergonomics – By doing the heavy lifting, hoisting equipment takes the strain off operators, reducing fatigue and lowering the risk of injury.
Flexible – Hoisting equipment are flexible and can be easily re-fitted for longer lifting capability.
Indoor/outdoor use – Hoisting equipment can be used with equipment inside a facility and also outside in the yard.
Lower maintenance costs – Hoisting equipment requires less maintenance compared to other lifting devices because they incorporate the latest technologies and offer a variety of usage and capacity ratings.
Portable – Hoisting equipments are portable and can be easily transported from job to job or site to site.
Positioning – Hoisting equipments are part of highly automated systems maneuver with the precision of one thousandth of the rated speed to an exact location.
Reduction in product damage – Hoisting equipments allow smooth, direct-path lifting over obstacles since they have soft start features, multiple speed options and a variety of end effectors to interface with and secure the load and hence products are handled gently to minimize damage.
Safety – Since hoisting equipments operate overhead and work in a specific area, they are less likely than forklift traffic to maneuver a load into personnel, walls, machinery or other obstacles.