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A recirculating-ball nut and screw set mounted with a lubrication device effective in improving the sliding characteristic and also realizing maintenance-free lubrication. The lubrication device is mainly comprised of a lubricating plate accommodated in a holder attached to a ball nut. The lubricati

A recirculating-ball nut and screw set mounted with a lubrication device effective in improving the sliding characteristic and also realizing maintenance-free lubrication. The lubrication device is mainly comprised of a lubricating plate accommodated in a holder attached to a ball nut. The lubricating plate is made with a tongue that will come into sliding engagement with a helical groove formed around a screw shaft. The lubricating plate is made of a sintered resinous component of porous structure in which cells are impregnated with lubricant.

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A recirculating-ball nut and screw set mounted with a lubrication device effective in improving the sliding characteristic and also realizing maintenance-free lubrication. The lubrication device is mainly comprised of a lubricating plate accommodated in a holder attached to a ball nut. The lubricati

A recirculating-ball nut and screw set mounted with a lubrication device effective in improving the sliding characteristic and also realizing maintenance-free lubrication. The lubrication device is mainly comprised of a lubricating plate accommodated in a holder attached to a ball nut. The lubricating plate is made with a tongue that will come into sliding engagement with a helical groove formed around a screw shaft. The lubricating plate is made of a sintered resinous component of porous structure in which cells are impregnated with lubricant. y of the gas corresponding to the speed of sound and the first and second, thermal conductivities. 2. A method as claimed in claim 1, in which the relative density is obtained by a procedure involving use of the formula: RD=g·ThCH+h·ThCL+i·SoS+j·Ta+k·Ta2+l, where CV is the relative density of the gas, where ThCHis the first thermal conductivity of the gas at said first temperature, where ThCLis the second thermal conductivity of the gas at said second temperature which is lower than said first temperature, where SoS is the speed of sound in gas at ambient temperature, and where Tais the ambient temperature of said gas whereof said thermal conductivities are measured, the first and second temperatures being greater than said ambient temperature, and g, h, i, j, k and l are constants. 3. A method as claimed in claim 2, in which SoS is the speed of sound in m/s, the thermal conductivities are in units of Watts/meter×degrees Kelvin (W/m·K), the temperature Taand the first and second temperatures are in degrees Celsius, and the relative density is in megajoules/standard cubic meter (MJ/m3st). 4. A method as claimed in claim 3, in which the gas is natural gas comprising at least one hydrocarbon gas which is methane, and at least one of nitrogen and carbon dioxide. 5. A method as claimed in claim 4, in which: g is substantially 0.017955, h is substantially -0.02812, i is substantially -0.00189, j is substantially 0.001807, k is substantially -0.0000026, and l is substantially 1.73041. 6. A method as claimed in claim 2, in which the gas is fuel gas. 7. A method as claimed in claim 6, in which the fuel gas is natural gas. 8. A method as claimed in claim 2, in which the first temperature is substantially 70° C. above ambient temperature. 9. A method as claimed in claim 2, in which the second temperature is substantially 50° C. above the ambient temperature. 10. A method of measuring the Wobbe index of gas using the formula in which WI is the Wobbe index, CV is the calorific value of the gas, and RD is the relative density obtained by the method as claimed in claim 1. 11. An apparatus to measure the relative density of a gas, comprising: means for measuring a speed of sound of the gas; means for measuring a first thermal conductivity of the gas at a first temperature; means for measuring a second thermal conductivity of the gas at a second temperature which differs from the first temperature; and means for using the speed of sound and the first and second thermal conductivities in an operation producing the relative density of the gas corresponding to said speed of sound and said first and second thermal conductivities. 12. An apparatus to measure the Wobbe index of gas using the formula in which WI is the Wobbe index, CV is the calorific value of the gas and RD is the relative density of the gas obtained using an apparatus as claimed in claim 11. 13. An apparatus to measure the relative density of a gas, comprising: means for measuring a speed of sound of the gas; means for measuring a first thermal conductivity of the gas at a first temperature; means for measuring a second thermal conductivity of the gas at a second temperature which differs from the first temperature; and means for using the speed of sound and the first and second thermal conductivities in an operation producing the relative density of the gas corresponding to said speed of sound and said first and second thermal conductivities, in which the calorific value is obtained by a procedure involving use of the formula: RD=g·ThCH+h·ThCL+i·SoS+j·Ta+k·Ta2+l, where RD is the relative density of the gas, where ThCHis the first thermal conductivity of the gas at said first temperature, where ThCLis the second thermal conductivity of the