Quick Facts

  • Manufacturer: JOST International
  • Product Series: JSK37U
  • Product Type: Cast commercial-truck fifth wheel
  • Top-Plate Construction: Cast ductile iron
  • Maximum Vertical Load: 55,000 pounds
  • Maximum Horizontal Rating: 150,000 pounds
  • Kingpin Compatibility: Standard 2-inch SAE kingpin
  • Release Options: Standard manual, retractable-handle, and air-release configurations
  • Lubrication Options: Standard lubricated or JSK37UW low-lube top plate
  • Mounting Options: Compatible JOST sliding and stationary mounting systems
  • Primary Applications: Highway tractors, regional tractors, tanker fleets, flatbed operations, and general commercial trucking
  • Production Status: Current JOST North American fifth-wheel series

Overview

The JOST JSK37U is a cast-ductile-iron fifth wheel designed for commercial highway and vocational tractor applications. It is part of JOST’s primary North American fifth-wheel family and is available as an original-equipment specification from multiple truck manufacturers.

The series is designed around a standard 2-inch trailer kingpin and provides a maximum vertical-load rating of 55,000 pounds with a maximum horizontal rating of 150,000 pounds.

JOST offers the JSK37U in several operating configurations, including conventional manual release, air release, retractable handle, and low-lube versions. Different sliding and stationary mounts allow the fifth wheel to be matched to tractor-frame dimensions, installed height, weight-distribution needs, and fleet operating requirements.

Key Characteristics

  • Cast-ductile-iron top plate
  • 55,000-pound maximum vertical rating
  • 150,000-pound maximum horizontal rating
  • Standard 2-inch kingpin compatibility
  • Standard manual-release configuration
  • Optional air-release configuration
  • Retractable-handle options
  • Low-lube JSK37UW option
  • Replaceable locking and wear components
  • Sliding and stationary mounting options
  • Optional hardened steel throat wear ring
  • Broad North American truck-OEM availability

Major Components

  • Cast-ductile-iron top plate
  • Fifth-wheel throat
  • Lock jaw
  • Lock bar
  • Cushion or wear ring
  • Release handle and linkage
  • Secondary locking components
  • Pivot pins
  • Mounting brackets
  • Slider rails and locking plungers when equipped
  • Optional air-release cylinder
  • Optional low-lube top-plate liners
  • Lubrication fittings and grease channels

Coupling Type and Design

The JSK37U is a conventional semi-oscillating fifth wheel designed for a standard 2-inch SAE trailer kingpin. The cast top plate pivots forward and rearward to accommodate normal tractor-trailer articulation.

The coupling mechanism uses a lock jaw and lock bar to capture and retain the kingpin. Replaceable wear components allow the fifth wheel to be serviced without automatically replacing the complete top plate.

JOST offers different JSK37U top-plate configurations so fleets can select conventional lubrication, low-lube operation, manual release, retractable release, or pneumatic release.

How the Coupling System Works

As the tractor backs beneath the trailer, the trailer kingpin enters the fifth-wheel throat. The kingpin moves into the locking mechanism, causing the lock jaw and lock bar to close around it.

Once correctly coupled, the fifth wheel:

  • Supports the front of the trailer
  • Retains the trailer kingpin
  • Transfers pulling and braking forces
  • Permits normal tractor-trailer articulation
  • Transfers trailer weight into the tractor frame and axles

The driver must confirm the connection through a controlled pull test and direct visual inspection. Handle position alone does not prove that the kingpin is correctly locked.

Position and Function Within the Vehicle

The JSK37U mounts above the tractor frame and rear-axle group. It serves as the primary mechanical connection between the tractor and a compatible semitrailer.

Its position allows the trailer’s imposed load to be distributed through the mounting system, tractor frame, suspension, axles, tires, and wheels. A sliding mount allows this load distribution to be adjusted by moving the fifth wheel forward or rearward.

Common Specifications

  • Top-Plate Material: Cast ductile iron
  • Maximum Vertical Load: 55,000 pounds
  • Maximum Horizontal Load: 150,000 pounds
  • Kingpin Size: Standard 2-inch SAE kingpin
  • Standard Release: Manual operating handle
  • Optional Release: Air release or retractable-handle configuration
  • Top-Plate Lubrication: Conventional grease on standard JSK37U models
  • Low-Lube Option: JSK37UW replaceable low-friction top-plate liners
  • Mounting: Compatible JOST sliding or stationary mounting system
  • Articulation: Forward-and-rear top-plate pivoting

Mounting height, slide travel, assembly weight, release system, bracket design, and lubrication configuration depend on the complete model and assembly number.

Load Ratings and Operating Capacity

JOST rates the JSK37U Series for a maximum vertical load of 55,000 pounds and a maximum horizontal load of 150,000 pounds.

The fifth-wheel rating does not independently determine the capacity of the complete tractor-trailer combination. Actual capacity remains limited by the lowest-rated component, including:

  • Fifth-wheel mounting system
  • Tractor frame
  • Axles and suspension
  • Tires and wheels
  • Trailer kingpin
  • Trailer upper coupler
  • Trailer structure
  • Certified vehicle ratings

Applications exceeding the JSK37U’s ratings require a properly selected higher-capacity coupling system.

Articulation and Oscillation

The JSK37U pivots forward and rearward to accommodate normal highway grades, loading-dock approaches, ramps, and tractor-trailer movement.

It is not a fully oscillating fifth wheel and does not provide unrestricted side-to-side movement. Specialized heavy-haul or severe cross-axle terrain may require a different JOST coupling.

Sliding and Adjustment System

The JSK37U can be combined with compatible JOST sliding mounting systems. Moving the fifth wheel forward or rearward can change:

  • Steering-axle loading
  • Drive-axle loading
  • Tractor-to-trailer clearance
  • Trailer swing clearance
  • Landing-gear clearance
  • Overall weight distribution

Slide travel, adjustment increments, installed height, and lock design depend on the selected mounting assembly. Every slider lock must be fully engaged and visually inspected before road operation.

Release and Locking Mechanism

The JSK37U uses a lock jaw and lock bar to retain the trailer kingpin. The design allows the condition of critical locking components to be checked during inspection and serviced with approved replacement parts.

Available release arrangements may include:

  • Standard external manual handle
  • Retractable manual handle
  • Air-operated release mechanism
  • Application-specific handle orientation

An air or retractable release improves convenience but does not replace a visual lock inspection or controlled pull test.

Air and Electronic Controls

Standard manual-release JSK37U models do not require air or electronic controls. Air-release configurations require:

  • A compatible tractor air supply
  • Protected air lines and fittings
  • A release-control valve
  • A pneumatic actuator
  • Correctly adjusted mechanical linkage
  • Appropriate safety interlocks when specified

JOST also offers coupling-status and illumination technology for certain applications. Compatibility must be verified using the complete fifth-wheel model and control-system information.

Mounting and Installation

The JSK37U must be installed with an approved mounting system and procedures appropriate for the tractor frame, fifth-wheel rating, trailer, and operating application.

Installation must provide:

  • Adequate tractor-frame strength
  • Correct mounting height
  • Approved mounting fasteners
  • Secure bracket attachment
  • Sufficient trailer swing clearance
  • Landing-gear and tire clearance
  • Correct slider travel and stops
  • Protected air-line routing when equipped
  • Access for inspection and service

Vehicle Frame and Mounting Compatibility

Compatibility depends on:

  • Tractor manufacturer and model
  • Frame width and rail construction
  • Rear-axle arrangement
  • Suspension configuration
  • Required fifth-wheel height
  • Available mounting length
  • Required slider travel
  • Trailer swing and landing-gear clearance
  • Completed vehicle weight distribution
  • Intended operating duty

A JSK37U top plate should not be fitted to an unidentified or unapproved mounting system solely because the pivot-pin dimensions appear compatible.

Mating Component and Trailer Compatibility

The JSK37U must be paired with an approved standard 2-inch SAE kingpin and compatible trailer upper-coupler plate.

The following mating components and conditions must be inspected:

  • Kingpin size and profile
  • Kingpin wear and damage
  • Upper-coupler flatness
  • Upper-coupler surface condition
  • Trailer structural condition
  • Trailer load rating
  • Swing clearance
  • Landing-gear clearance

Common Applications

  • Over-the-road tractors
  • Regional-haul tractors
  • Dry-van operations
  • Refrigerated-freight operations
  • Tanker tractors
  • Flatbed operations
  • Approved vocational tractors
  • General commercial-trucking fleets

Advantages

  • Durable cast-ductile-iron construction
  • 55,000-pound vertical-load rating
  • 150,000-pound horizontal rating
  • Multiple release configurations
  • Standard and low-lube options
  • Sliding and stationary mounting choices
  • Replaceable locking and wear components
  • Optional hardened steel throat wear ring
  • Broad truck-OEM availability
  • Established service and replacement-parts support

Limitations or Considerations

  • Not intended for loads exceeding its published ratings
  • Standard models require regular top-plate lubrication
  • Low-lube liners are replaceable wear components
  • Slider systems add inspection and maintenance points
  • Does not provide full side-to-side oscillation
  • Air-release configurations require a reliable air supply
  • Mounting height and slide travel must match the application
  • Does not increase the tractor’s certified ratings
  • Replacement parts must match the exact JSK37U configuration

Driver and Operator Experience

A correctly maintained JSK37U provides predictable coupling, articulation, and trailer response. Air-release and retractable-handle configurations can reduce the effort required to reach and operate the release mechanism.

Drivers must still verify the connection through a pull test and visual inspection. On sliding assemblies, every locking plunger must be engaged before the truck returns to service.

Coupling and Uncoupling Procedure

Before coupling:

  • Inspect the top plate and locking mechanism
  • Confirm the lock is open
  • Inspect the kingpin and upper coupler
  • Verify correct trailer height
  • Check top-plate lubrication or low-lube liners
  • Confirm the mounting system is secure

After coupling:

  • Perform a controlled pull test
  • Visually inspect the lock around the kingpin
  • Confirm the release mechanism returned to its locked position
  • Verify there is no gap between the mating plates
  • Confirm slider locks are engaged when equipped
  • Connect and inspect air and electrical lines
  • Raise and secure the landing gear

These steps are an operational overview and do not replace JOST instructions or fleet safety procedures.

Inspection and Wear Points

  • Lock jaw and lock bar
  • Kingpin contact area
  • Cushion or wear ring
  • Fifth-wheel throat
  • Release handle and linkage
  • Top-plate surface
  • Low-lube liners when equipped
  • Pivot pins and mounting brackets
  • Slider rails and locking plungers
  • Mounting fasteners and frame attachment points
  • Air lines and actuator when equipped
  • Trailer kingpin and upper coupler

Lubrication Requirements

Standard JSK37U models require lubrication across the top-plate surface and at the locking mechanism, pivot points, and other manufacturer-specified locations.

JSK37UW low-lube models use replaceable liners on the top-plate surface, but the lock jaw, lock bar, pivot points, and other internal components still require specified lubrication.

During cold-weather or contaminated service:

  • Remove hardened or contaminated lubricant
  • Clean the locking mechanism
  • Inspect wear components
  • Apply the approved lubricant
  • Confirm free lock and release operation

Maintenance and Ownership Notes

Routine maintenance should include:

  • Cleaning and lubricating standard top plates
  • Inspecting low-lube liners when equipped
  • Checking the lock jaw and lock bar
  • Measuring kingpin and lock wear
  • Inspecting the cushion or wear ring
  • Checking the release handle and linkage
  • Inspecting pivot pins and brackets
  • Checking slider rails and locks
  • Testing air-release controls when equipped
  • Inspecting mounting fasteners and tractor frame

The complete model, serial, top-plate, and mounting information should be recorded before ordering service parts or rebuild kits.

Common Coupling Problems

  • High coupling: Incorrect trailer height allows the kingpin to pass above the locking mechanism instead of entering through the throat.
  • Failure to lock: Dirt, damage, binding, poor lubrication, incorrect trailer height, or worn components may prevent engagement.
  • Difficult release: Pressure against the lock, damaged linkage, corrosion, contamination, or incorrect adjustment may restrict the handle.
  • Excessive kingpin movement: A worn kingpin, lock jaw, lock bar, cushion ring, or adjustment component may create excessive play.
  • Rapid throat wear: Repeated coupling impacts, contamination, misalignment, or a damaged kingpin may wear the throat or cushion ring.
  • Slider will not move: Low air pressure, damaged lines, contaminated rails, worn components, or tractor-trailer binding may prevent adjustment.
  • Air-release failure: Low pressure, damaged air lines, a failed actuator, or binding linkage may prevent pneumatic release.
  • Low-lube liner damage: Contamination, an uneven upper coupler, raised fasteners, or incorrect replacement parts may cause premature wear.

Safety and Service Precautions

Never rely on release-handle position alone to confirm coupling. Before moving the vehicle:

  • Perform a controlled pull test
  • Inspect the lock around the kingpin
  • Confirm the release system is fully locked
  • Check for a gap between the mating plates
  • Verify slider locks are engaged when equipped

Never place any part of the body between unsecured vehicles or beneath an unsupported trailer. Do not weld, heat, grind, drill, straighten, or modify structural components unless specifically permitted by JOST.

Identification Information

Record the following information before ordering parts or a replacement assembly:

  • JOST JSK37U model designation
  • Complete assembly number
  • Top-plate part number
  • Serial or production information
  • Standard or low-lube configuration
  • Manual, retractable, or air-release configuration
  • Sliding or stationary mounting type
  • Mounting height
  • Slider travel and adjustment increments
  • Vehicle identification number
  • Kingpin size and condition

Do not identify replacement components solely by appearance. Different JSK37U configurations may use different handles, liners, actuators, brackets, and slider components.

Buying Considerations

  • Confirm vertical and horizontal load requirements
  • Choose standard or low-lube operation
  • Select manual, retractable, or air release
  • Choose sliding or stationary mounting
  • Confirm mounting height and slider travel
  • Verify tractor-frame compatibility
  • Check trailer swing and landing-gear clearance
  • Confirm kingpin and upper-coupler compatibility
  • Review assembly weight and payload requirements
  • Confirm genuine service-parts availability
  • Use current JOST application and installation information

Variants and Configurations

Common JSK37U Series configurations include:

  • JSK37U standard manual-release top plate
  • JSK37U A air-release top plate
  • JSK37U B retractable-handle top plate
  • JSK37UW low-lube top plate
  • Sliding mounting assemblies
  • Stationary mounting assemblies
  • Different mounting heights
  • Different slider travels
  • Optional hardened steel throat wear ring
  • Application-specific handle orientations

Complete model and assembly numbers must be verified because components are not automatically interchangeable across every JSK37 configuration.

Comparison to Similar Equipment

Compared with the Fontaine No-Slack 7000 Fifth Wheel, the JSK37U uses JOST’s lock-jaw and lock-bar design and JOST-specific mounting systems.

Compared with the Holland FW35 Fifth Wheel, the JSK37U provides similar general commercial-truck capacity but uses a different locking mechanism, wear system, and mount design.

Compared with the Fontaine No-Slack 7000CC Clean Connect Fifth Wheel, the JSK37UW offers JOST’s approach to a reduced-lubrication top-plate surface.

Compared with the JOST JSK36N Lightweight Fifth Wheel, the cast JSK37U has a higher vertical-load rating and broader application range.

Best Fit For

  • Standard and heavy-duty highway tractors
  • Regional and long-haul fleets
  • Dry-van and refrigerated operations
  • Tanker and flatbed fleets
  • Fleets requiring manual or air release
  • Operators wanting a low-lube option
  • Applications requiring sliding or stationary mounting
  • Fleets using trucks with factory JOST fifth-wheel availability

Related Vehicle and Trailer Types

Related Truck and Trailer Models

Related Coupling Types

Related Coupling Products

Related Equipment Systems

Related Guides

Regulatory and Compliance Notes

The fifth wheel, mounting system, kingpin, upper coupler, tractor, and trailer must comply with applicable commercial-vehicle construction and operating requirements.

The JSK37U’s mechanical capacity does not override:

  • Gross axle weight ratings
  • Gross vehicle weight ratings
  • Gross combination weight ratings
  • Frame and suspension ratings
  • Tire and wheel ratings
  • Kingpin and upper-coupler ratings
  • Federal, state, provincial, or local weight limits

Real World Operating Notes

The JSK37U’s multiple release and lubrication options allow fleets to standardize the basic fifth-wheel family while selecting different configurations for specific tractors.

Low-lube models reduce grease on the top-plate surface but do not eliminate internal lubrication. Air-release systems improve convenience but must remain correctly adjusted and supplied with clean compressed air.

When a slider is repositioned, drivers should inspect every lock and confirm axle weights on a certified scale. A slider’s position should never be judged solely by the in-cab control.

Frequently Asked Questions

What is the JOST JSK37U?

It is a cast-ductile-iron fifth wheel designed for standard commercial highway and vocational tractor applications.

What are the JSK37U load ratings?

JOST rates the series for a maximum vertical load of 55,000 pounds and a maximum horizontal load of 150,000 pounds.

What kingpin does the JSK37U use?

It is designed for a standard 2-inch SAE semitrailer kingpin.

Is an air release available?

Yes. JOST offers air-release JSK37U configurations as well as standard and retractable manual handles.

Is there a low-lube version?

Yes. The JSK37UW uses replaceable low-friction liners on the top-plate surface.

Can the JSK37U be installed on a slider?

Yes. It can be paired with compatible JOST sliding or stationary mounting assemblies.

Does the JSK37U require lubrication?

Standard models require top-plate and internal lubrication. Low-lube versions reduce top-surface greasing but still require lubrication of the lock and other specified components.

How are the correct replacement parts identified?

Use the complete model, top-plate number, serial information, release type, lubrication configuration, mounting system, installed height, and slide travel.

Availability and Where to Find

JOST JSK37U assemblies, replacement top plates, mounts, rebuild kits, lock jaws, lock bars, wear rings, low-lube liners, handles, and air-release components may be available through JOST dealers, truck manufacturers, commercial-truck dealerships, heavy-duty parts suppliers, and fifth-wheel service facilities.

Availability depends on release type, lubrication configuration, mounting height, slide travel, and complete assembly number.

Current product details are available through the official JOST JSK37U Series page. Manuals, parts information, and technical literature are available through the JOST literature library.

Support Trucking Families

Truck Equipment Atlas supports the mission of Grey Cross, a nonprofit dedicated to helping families of truck drivers who lose their lives while on the road. As the site grows, part of its mission is to bring awareness and support to these families.

Podcast or Video Section

Learn More Through Audio and Video

Truck Equipment Atlas will continue expanding with podcast episodes, interviews, and equipment breakdowns to help drivers and fleets better understand trucks and equipment.

Follow the Growth of Truck Equipment Atlas

Truck Equipment Atlas continues to expand with new truck profiles, trailer coverage, diesel engine pages, equipment system resources, manufacturer information, and educational guides.

Visit our Site Updates page to explore recent additions, project milestones, and upcoming resources as we continue building one of the industry’s growing commercial trucking equipment reference libraries.

View the Latest Site Updates: Truck Equipment Atlas Site Updates

Scroll to Top